iommu/arm-smmu: Disable clocks in reverse order

Currently we're disabling clocks in the same order in which we enable
them, but some clocks have dependencies and ordering requirements so we
should actually be disabling them in reverse order.  Do it.

CRs-Fixed: 1000848
Change-Id: Ie01df24b72a3247a24ab6fbd7a90ec8cfee7236f
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>