clk: qcom: clk-branch: 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: Iabed22d492c4445e7cc97261671c1ea4240bed71
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>