msm: clock-generic: Add a get_rate op for slave dividers

Clk_get_rate on slave dividers currently returns the cached
rate. However, in case of some fixed dividers, the parent
rate may change without set_rate ever being called on the
slave divider clock. Therefore, add a get_rate op that
divides the parent clock rate and returns the result.

Change-Id: I94dbd7238b3cd4538f8ffaea6a1a146b4432704c
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>