This commit:

commit 8de000cf0265eaa4f63aff9f2c7a3876d2dda9b6
Author: Yong Wu <yong.wu@mediatek.com>
Date:   Fri Mar 26 11:23:36 2021 +0800

    iommu/mediatek-v1: Allow building as module

changes drivers/iommu/Kcconfig

 config MTK_IOMMU_V1
-       bool "MTK IOMMU Version 1 (M4U gen1) Support"
+       tristate "MediaTek IOMMU Version 1 (M4U gen1) Support"

Unfortunately, this doesn't actually build properly as a module, because:

  MODPOST modules-only.symvers
ERROR: modpost: "of_phandle_iterator_args" [drivers/iommu/mtk_iommu_v1.ko] undefined!
make[2]: *** [/usr/src/linux-next/scripts/Makefile.modpost:150: modules-only.symvers] Error 1

There's no EXPORT_SYMBOL and drivers/of/base.o is a built-in, so
the symbol isn't available to modules.