clk: msm: clock-local2: Add support for branch clocks rate aggregation

There are cases when multiple branch clocks need to be able to
scale their RCG independently. The RCG should then be configured
to run at the maximum rate of all it's children clocks which are
enabled at that point. Add support for this.

Change-Id: I90b7a9a3007792f65e0292d375e409ce1dbf0c08
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>