msm: iommu: Rework clock logic and add IOMMU bus clock control

Clean up the clock control code in the probe calls, and add
support for controlling the clock for the IOMMU bus
interconnect. With the (proper) clock driver in place, the
clock control logic in the probe function can be made much
cleaner since it does not have to deal with the placeholder
driver anymore.

Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>
Reviewed-by: Trilok Soni <tsoni@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>