powerpc/pseries: Optimize IOMMU setup

The previous commit will use the page-at-a-time hypervisor call for
setting up IOMMU entries when we are using 64k pages and setting up
one 64k page, even though that means 16 calls to the hypervisor, since
the hypervisor still works on 4k pages.  This optimizes this case by
using the multi-page IOMMU setup hypervisor call instead.

Signed-off-by: Paul Mackerras <paulus@samba.org>