AMD Opteron and Athlon 64 processors have an AGP aperture
and GART built into the processor.  Linux has used the
AGP GART as an IOMMU to improve the performance of 32-bit
only PCI devices when DMA'ing into physical memory above
0xffffffff.  This patch provides a similar capability 
for Xen dom0.

Most of the code simply migrates the native Linux
aperture.c and pci-gart.c to dom0.  A new memory op
is added to clear the aperture mapping from the 
hypervisor's page tables, which is necessary to
prevent cache alias issues resulting from processor
speculation.

My thanks to Ulrich Meis [um@amd64.org], who was 
instrumental in developing and testing this patch. 

Signed-off-by: Mark Langsdorf <mark.langsdorf@amd.com>