msm: clock: Add pre_set_rate and post_set_rate ops

Some clocks might need to perform additional steps before/after a voltage
changes happen during a rate change. To accomodate such clocks and similar
requirements from new clock types in the future, add generic
pre/post_set_rate ops that are called before any HW state change is done
for set rate.

Change-Id: I07e4f79136028aaae5dee6d66d18b45c445f325b
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>