msm: iommu: Convert to clk_prepare/unprepare

Add calls to clk_prepare and unprepare so that MSM can migrate to
the common clock framework. We never unprepare the clocks until
driver remove because the clocks are enabled and disabled in irq
context. Finer grained power management is possible in the future
via runtime power management techniques.

Cc: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Joerg Roedel <joro@8bytes.org>
Signed-off-by: David Brown <davidb@codeaurora.org>