[IA64] ensure XPC and XPNET are loaded on sn2 platforms only

These are SN2 only drivers.  They should have platform checks to prevent
them from doing evil stuff in GENERIC kernels.

Signed-off-by: Martin Hicks <mort@sgi.com>
Acked-by: Dean Nelson <dcn@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>