sgi-xp: define xp_expand_memprotect() and xp_restrict_memprotect()

Define xp_expand_memprotect() and xp_restrict_memprotect() so they can be
tailered to the hardware they are run on.

Signed-off-by: Dean Nelson <dcn@sgi.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>