msm: clock-local2: Add support to return a rounded rate for branch clocks

Branch clocks that do not have siblings can propagate
set-rate calls to their parents. There is no reason
this can't be extended to implement the API to return
a rounded rate that can be set on these branch clocks.
Implement branch_clk_round_rate.

Change-Id: I3502168ea089d4736660a0cb09b5a62448df973a
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>