iommu/arm-smmu: program implementation defined registers on attach

Some platforms require certain implementation-defined registers to be
programmed when first attaching to the SMMU. Add support for this via
specifying register offset, value pairs in the device tree.

Change-Id: Iac2fe42684c3849a24d0d1251a206954262257c5
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>