iommu: Support dynamic pgsize_bitmap

Currently we use a single pgsize_bitmap per IOMMU driver.  However, some
IOMMU drivers might service different IOMMUs with different supported
page sizes.  Some drivers might also want to restrict page sizes for
different use cases.  Support these use cases by adding a
.get_pgsize_bitmap function to the iommu_ops which can optionally be
used by the driver to return a domain-specific pgsize_bitmap.

CRs-Fixed: 997751
Change-Id: I46d70733be647599e148fe52258a4d8f009ac48a
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>