x86: uv: xpc needs to provide an abstraction for uv_gpa

Provide an SGI SN2/UV agnositic method for converting a global physical
address into a socket physical address.

Signed-off-by: Robin Holt <holt@sgi.com>
Cc: Jack Steiner <steiner@sgi.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>