iommu: Add iommu_iova_to_phys_hard

Some IOMMU hardware implementations provide hardware translation
operations that can be useful during debugging and development.  Add a
function for this purpose along with an associated op in the iommu_ops
structure so that drivers can implement it.

Change-Id: I54ad5df526cdce05f8e04206a4f01253b3976b48
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>