Add an AGP driver for the AGP aperture in the northbridge of the AMD Hammer.
The AGP driver works for both 32bit and 64bit kernels.

It also adds some hooks to the AGP driver to allow the x86-64 GART based
IOMMU code to share the aperture with AGP. The hooks are intentionally kept
minimalistic. In addition it needs some Config.in hackery, because AGP cannot
be modular in this case, because the IOMMU needs to control its startup and
it runs early when PCI is initialized.

The original AGP driver was done by Dave Jones, I added the IOMMU support.