iommu/arm-smmu: implement generic DT bindings

Generic IOMMU device tree bindings were recently added in [bb1e25b9ed:
"devicetree: Add generic IOMMU device tree bindings"]. Implement the
bindings in the ARM SMMU driver.

See Documentation/devicetree/bindings/iommu/iommu.txt for the bindings
themselves.

Change-Id: I318b4cc347caccf3e142ce021057599eaea07814
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>