msm: clock-rpm: Remove rpm_branch_clk_get_rate()

The commit that added support for RPM branch clocks (a6835112)
won a merge race with a commit that eliminated the need for
single-rate clocks to specify .get_rate function ops (7949ecb3).
Eliminate the custom .get_rate op and use the rate member of
struct clk to store the rate instead.

This makes the clk_get_rate() implementation for RPM branch
clocks similar to that of other fixed-rate clocks.

Change-Id: I49efb541727c71f1617d43b3e7085e848427043d
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>