msm: clock-generic: Allow separate enable/mux-select registers

Some muxes may have independent registers to control
mux selection and enable/disable. Add an additional
field to the mux clock struct to support this.

Change-Id: I436ca0ba48d186fb76d4e5585be4dd1f41fe7739
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>