x86/PCI: Detect mmconfig on nVidia MCP55

Detect and enable memory-mapped PCI configuration space on the nVidia
MCP55 southbridge.  Tested against 2.6.27.4 on an Arista Networks
development board with one MCP55, Coreboot firmware, no ACPI.

Signed-off-by: Ed Swierk <eswierk@aristanetworks.com>
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>