msm: clock-a7: Add functions to implement mux_div ops for a7

The mux_div clk type uses function pointers to implement the nongeneric
aspects of its functionality. Add functions which allow reading and
writing to the necessary registers.

Change-Id: Ic2abe91bfd5f9de03d48e9b3a313fb7068ca71b5
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>