iommu: arm-smmu: Allow configurations with zero clocks

If no clocks are present in devicetree, ensure that smmu->num_clocks is
set to zero instead of an error value.

Change-Id: Ibc3ede250419dd0235f3aa481218e3a91cdb3630
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>