msm: cpr-regulator: Add support to write to RBCPR CLK select register

In order for CPR to operate properly, it needs to be clocked from
a 19.2 MHz source.  Add the ability to write to the RBCPR CLK
select register so that the 19.2 MHz XO clock is always used.

If a faster clock is used, then it is likely that CPR will
recommend a supply voltage that is above the ceiling of a given
performance corner.

Change-Id: I1ee131f1260197e8aa117f1c307600ad5a9321f8
Signed-off-by: David Collins <collinsd@codeaurora.org>