msm: clock-rpm: Abstract out calls to RPM APIs.

The RPM communication driver is changing its underlying
transport layer for upcoming chipsets. To support
the current transport layer and future ones, abstract
out the calls to the RPM APIs into helper functions.

Change-Id: I7bd2706dd7bb69812b3baac12eca7893aca749ab
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>