iommu: Add {enable,disable}_config_clocks ops

There are certain use cases where it might be necessary to leave the
IOMMU's configuration clocks on.  This might happen in places where an
IOMMU's clocks might not be known.  A good example of this would be a
test library that needs to be able to do TLB invalidation from atomic
context.  It would need to enable clocks up front (outside of atomic
context) and leave them on for the duration of the test.

Add some ops for enabling and disabling configuration clocks.

CRs-Fixed: 997751
Change-Id: I95056952f60494fe5745f2183f9af8aab3a40315
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>