iommu/iommu-debug: Add debugfs file to enable config clocks

It's fairly common while debugging to need to enable the config clocks
for an SMMU so that you can poke around at the registers.  Add a debugfs
file to do this.

CRs-Fixed: 997751
Change-Id: I31b90d64c2facb0a681f9da586e2c90803776819
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>