So I did a 'make allmodconfig' and then a 'make' on an RPi4 ARM box, and it
decided that CONFIG_SPARSEMEM=n was OK (so an include of linux/mmzone.h doesn't
define some needed values).

The offending code in resource.c is wrapped in a #ifdef CONFIG_DEVICE_PRIVATE,
which throws a whinge during 'make menuconfig' or 'make allmodconfig':

WARNING: unmet direct dependencies detected for DEVICE_PRIVATE
  Depends on [n]: ZONE_DEVICE [=n]
  Selected by [m]:
  - DRM_NOUVEAU_SVM [=y] && HAS_IOMEM [=y] && DRM_NOUVEAU [=m] && MMU [=y] && STAGING [=y]

after which I end up with CONFIG_DEVICE_PRIVATE=y in the .config file.

make menuconfig tells me:
Symbol: ZONE_DEVICE [=n]
   Type  : bool
   Defined at mm/Kconfig:779
     Prompt: Device memory (pmem, HMM, etc...) hotplug support
     Depends on: MEMORY_HOTPLUG [=n] && MEMORY_HOTREMOVE [=n] && SPARSEMEM_VMEMMAP [=n] && ARCH_HAS_PTE_DEVMAP [=n]
     Location:
   (1) -> Memory Management options
   Selects: XARRAY_MULTI [=n]

Pretty obviously a Kconfig whoops, but I have no idea what the proper Kconfig
fix is for this..

May be related to:

commit 0092908d16c604b8207c2141ec64b0fa4473bb03
Author: Christoph Hellwig <hch@lst.de>
Date:   Wed Jun 26 14:27:06 2019 +0200

    mm: factor out a devm_request_free_mem_region helper

which added the #ifdef CONFIG_DEVICE_PRIVATE code in question, except that's a
pretty old commit...  The only thing I'm sure of is that DEVICE_PRIVATE=y and
SPARSEMEM=n blows up. :)

  CC      kernel/resource.o
In file included from ./include/linux/cache.h:5,
                 from ./include/linux/printk.h:9,
                 from ./include/linux/kernel.h:15,
                 from ./include/asm-generic/bug.h:19,
                 from ./arch/arm/include/asm/bug.h:60,
                 from ./include/linux/bug.h:5,
                 from ./include/linux/mmdebug.h:5,
                 from ./include/linux/gfp.h:5,
                 from ./include/linux/slab.h:15,
                 from kernel/resource.c:17:
kernel/resource.c: In function '__request_free_mem_region':
kernel/resource.c:1653:28: error: 'PA_SECTION_SHIFT' undeclared (first use in this function); did you mean 'SECTION_SHIFT'?
  size = ALIGN(size, 1UL << PA_SECTION_SHIFT);
                            ^~~~~~~~~~~~~~~~
./include/uapi/linux/kernel.h:11:47: note: in definition of macro '__ALIGN_KERNEL_MASK'
 #define __ALIGN_KERNEL_MASK(x, mask) (((x) + (mask)) & ~(mask))
                                               ^~~~
./include/linux/kernel.h:33:22: note: in expansion of macro '__ALIGN_KERNEL'
 #define ALIGN(x, a)  __ALIGN_KERNEL((x), (a))
                      ^~~~~~~~~~~~~~
kernel/resource.c:1653:9: note: in expansion of macro 'ALIGN'
  size = ALIGN(size, 1UL << PA_SECTION_SHIFT);
         ^~~~~
kernel/resource.c:1653:28: note: each undeclared identifier is reported only once for each function it appears in
  size = ALIGN(size, 1UL << PA_SECTION_SHIFT);
                            ^~~~~~~~~~~~~~~~
./include/uapi/linux/kernel.h:11:47: note: in definition of macro '__ALIGN_KERNEL_MASK'
 #define __ALIGN_KERNEL_MASK(x, mask) (((x) + (mask)) & ~(mask))
                                               ^~~~
./include/linux/kernel.h:33:22: note: in expansion of macro '__ALIGN_KERNEL'
 #define ALIGN(x, a)  __ALIGN_KERNEL((x), (a))
                      ^~~~~~~~~~~~~~
kernel/resource.c:1653:9: note: in expansion of macro 'ALIGN'
  size = ALIGN(size, 1UL << PA_SECTION_SHIFT);
         ^~~~~
In file included from ./include/asm-generic/bug.h:19,
                 from ./arch/arm/include/asm/bug.h:60,
                 from ./include/linux/bug.h:5,
                 from ./include/linux/mmdebug.h:5,
                 from ./include/linux/gfp.h:5,
                 from ./include/linux/slab.h:15,
                 from kernel/resource.c:17:
kernel/resource.c:1654:48: error: 'MAX_PHYSMEM_BITS' undeclared (first use in this function); did you mean 'MAX_UINSN_BYTES'?
  end = min_t(unsigned long, base->end, (1UL << MAX_PHYSMEM_BITS) - 1);
                                                ^~~~~~~~~~~~~~~~
./include/linux/kernel.h:848:40: note: in definition of macro '__typecheck'
   (!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
                                        ^
./include/linux/kernel.h:872:24: note: in expansion of macro '__safe_cmp'
  __builtin_choose_expr(__safe_cmp(x, y), \
                        ^~~~~~~~~~
./include/linux/kernel.h:940:27: note: in expansion of macro '__careful_cmp'
 #define min_t(type, x, y) __careful_cmp((type)(x), (type)(y), <)
                           ^~~~~~~~~~~~~
kernel/resource.c:1654:8: note: in expansion of macro 'min_t'
  end = min_t(unsigned long, base->end, (1UL << MAX_PHYSMEM_BITS) - 1);
        ^~~~~
./include/linux/kernel.h:872:2: error: first argument to '__builtin_choose_expr' not a constant
  __builtin_choose_expr(__safe_cmp(x, y), \
  ^~~~~~~~~~~~~~~~~~~~~
./include/linux/kernel.h:940:27: note: in expansion of macro '__careful_cmp'
 #define min_t(type, x, y) __careful_cmp((type)(x), (type)(y), <)
                           ^~~~~~~~~~~~~
kernel/resource.c:1654:8: note: in expansion of macro 'min_t'
  end = min_t(unsigned long, base->end, (1UL << MAX_PHYSMEM_BITS) - 1);
        ^~~~~
make[1]: *** [scripts/Makefile.build:273: kernel/resource.o] Error 1
make: *** [Makefile:1726: kernel] Error 2