msm: clock-pll: Add round_rate for table based plls

The round rate function is used by clocks which attempt to configure
themselves dynamically based on the rate the client requests and
the rates the parent clock supports. The round_rate api is used to
determine the rates supported by the parent clock.

Change-Id: I6214492d68ba738f9ab9d642310a6fe8d3862956
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>