sgi-xp: create a common xp_remote_memcpy() function

Create a common remote memcpy function that maps to what the hardware
booted supports.

Signed-off-by: Dean Nelson <dcn@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>