iommu/arm-smmu: Add support for dynamic bus scale voting

Some SMMU operations (like register access and ATOS) require downstream
buses to be enabled.  We've been getting lucky for a long time since
other clients have been voting for the buses in question, but recently
we've seen some unclocked accesses due to our missing vote.  Add support
for bus scale voting to the driver.  The voting parameters will be
provided later in the per-target device tree SMMU configuration.

Change-Id: I6b31c887ad817a6628f4234622c73dee046d6442
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>