x86/PCI: amd: factor out MMCONFIG discovery

This factors out the AMD native MMCONFIG discovery so we can use it
outside amd_bus.c.

amd_bus.c reads AMD MSRs so it can remove the MMCONFIG area from the
PCI resources.  We may also need the MMCONFIG information to work
around BIOS defects in the ACPI MCFG table.

Cc: Borislav Petkov <borislav.petkov@amd.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: stable@kernel.org       # 2.6.34+
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>