iommu/arm-smmu: add support for saving config registers

Some hardware is capable of retaining register values during power
collapse.  Add an option (configurable via DT) to enable this feature.
This is implemented by always enabling/disabling regulators every time
clocks are enabled/disabled.

Change-Id: I89d9a4f4a2eb29f0868b309d55a77cc4ed50e22e
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>