iommu/arm-smmu: Implement .trigger_fault

In order to facilitate debugging and development, implement the
.trigger_fault method from iommu_ops.  This can be done on ARM SMMUv2 by
writing to the FSRRESTORE register.  Do it.

Change-Id: Ia8339b54fbb9263d8cf23ff61c4615122316729a
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>