[IA64-SGI] volatile semantics in places where it seems necessary

Resend using accessors instead of volatile qualifiers per hch comments, and
easier to understand convenience macros per rja comments.

Patch to apply volatile semantics when accessing MMR's in various SN files.

Signed-off-by: Mark Maule <maule@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>