iommu: use correct clock names

Using clock_gcc worked while we were using dummy clocks,
but now clock_mmss must be used. Also, we were using venus
clock names for all IOMMUs since it didn't matter with dummy clocks,
but now it does.

Change-Id: I5f7872872d095c09c629d9da3bf23a5ddfe82567
Signed-off-by: Larry Bassel <lbassel@codeaurora.org>