So we can use MMCONF when MMCONF is not set by BIOS

using TOP_MEM2 msr to get memory top, and try to scan fam10h mmio routing to
make sure the range is not conflicted with some prefetch MMIO that is above 4G.
(current only LinuxBIOS assign 64 bit mmio above 4G for some co-processor)
Signed-off-by: Yinghai Lu <yinghai.lu@sun.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>