iommu/arm-smmu: Implement {enable,disable}_config_clocks

Some of our users need to be able to call
iommu_{enable,disable}_config_clocks on domains for SMMUs that we
control.  Implement them.

CRs-Fixed: 997751
Change-Id: Idc3692679409093faf8f458d53326e669d7f6479
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>