Impact: generalize the sw-IOTLB range checks

Some architectures require special rules to determine whether a range
needs mapping or not.  This adds a weak function for architectures to
override.
Signed-off-by: NIan Campbell <ian.campbell@citrix.com>
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>