sgi-xp: only build for ia64-sn2 when CONFIG_IA64_GENERIC specified

For the time being build for ia64-sn2 alone when CONFIG_IA64_GENERIC is
specified.

This eliminates a dependency of the XP/XPC drivers on having the GRU
driver insmod'd in order to insmod them, when running on an ia64-sn2
system.

On such a system the GRU driver serves no useful purpose.

Signed-off-by: Dean Nelson <dcn@sgi.com>
Cc: "Luck, Tony" <tony.luck@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>