iommu: Add client fault handler flag for external faults

Some IOMMUs (like the ARM SMMU) can raise "external faults" in addition
to translation and permission faults.  Add an IOMMU fault flag so that
this fault type can be propagated to client fault handlers.

Change-Id: Ia476ed6c727f5f01011747f30b3a74ebf0fc07f7
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>