iommu/arm-smmu: add support for DOMAIN_ATTR_CONTEXT_BANK

Because the ARM SMMU driver assigns context banks dynamically,
some drivers need a way to know which one they are using.

Change-Id: Ic0dedbad4327ef86c5a893a48b57f0f9417800e9
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>