[IA64-SGI] add sn_feature_sets bit

SGI's prom has added a new feature which avoids an Altix-specific
MCA that can occur with excessive use of ia64_pal_cache_flush.  This
patch adds the #define to the sn_feature_sets.h to reflect that bit
is taken.

Signed-off-by: Dean Roe <roe@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>