msm: iommu: Support alternate core clocks

Refactor the IOMMU clock control code to always require a
core clock as well as an interface clock. Add support for
an optional alternate core clock and update device tree
bindings accordingly. Clean up the probe function to remove
needless enabling / disabling of clocks.

Change-Id: I4d744ffabc1e6fb123bacfda324f64408257cb25
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>