sgi-xp: support runtime selection of xp_max_npartitions

Support runtime selection of the max number of partitions based on the
hardware being run on.

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>