iommu/arm-smmu: Only prepare clocks when necessary

arm_smmu_enable_regulators also prepares all of our clocks (similarly
for arm_smmu_disable_regulators), and is always called from
arm_smmu_enable_clocks.  arm_smmu_enable_clocks, also prepares our
clocks, so clocks are being prepared twice, which is once more than we
need.  Fix this by enabling (not preparing) clocks in
arm_smmu_enable_clocks, relying on arm_smmu_enable_regulators to prepare
the clocks beforehand.

Change-Id: Id07848f64a81522e27198d6e708159941b07d444
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>