iommu: Add iommu_trigger_fault

It can be useful to trigger an IOMMU fault during development and
debugging.  Add support to the IOMMU framework to do so.

Change-Id: I908c9f5b52c6abe937f031de546d290027ba64b5
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>