msm: thermal: Add support to update DPM voltage scale register

Register for a voltage change callback function
and adjust the DPM voltage scale register. DPM is
currently not aware of the voltage scaling change and
the voltage level that is changed to. This lets the
HLOS driver to notify the DPM about the voltage
scaling.

When voltage goes from low to high, program the register
after the change is applied. When voltage goes from high
to low, program the register before the change.

Change-Id: Icc46e4b1b461882d9fdd7833575316c62ee985ee
Signed-off-by: Jeff Bernard <jbernard@codeaurora.org>
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>