x86_64: check MSR to get MMCONFIG for AMD Family 10h

so even booting kernel with acpi=off or even MCFG is not there, we still can
use MMCONFIG.

Signed-off-by: Yinghai Lu <yinghai.lu@sun.com>
Cc: Andi Kleen <ak@suse.de>
Cc: Greg KH <greg@kroah.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>