[IA64] swiotlb abstraction (e.g. for Xen)

Add abstraction so that the file can be used by environments other than IA64
and EM64T, namely for Xen.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>