iommu/arm-smmu: add support for specifying regulators

On some power-constrained platforms it's useful to disable power when a
device is not in use. Add support for specifying regulators for SMMUs
and only leave power on as long as the SMMU is in use (attached).

Change-Id: I87191d325423f160ddd4b71f5bf3a92f4942b821
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>