clk: clock-rpm: Add round_rate op for rpm_branch_clk

The clock-generic code relies on clk_round_rate to get the closest
rate from the parent clock. Ensure that the rpm_branch_clk is
compatible.

Change-Id: Ia9c90e80ed39ffa278879d7af8a0ae826725c735
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>