clk: qcom: cpu: Add voltage voting for the CPU Mux

The CPU mux requires to vote correctly on the supply during the
frequency transitions, thus add the pre/post callback hooks which would
enable & set voltage vote on the rail.

Change-Id: I0d4bb88e8d319bfc3f4f8d682964eb7ff3719ce0
Signed-off-by: Taniya Das <tdas@codeaurora.org>