power: pm8921-bms: enable ccadc calibration

The ccadc needs to be calibrated every few minutes for accurate
battery charge estimation. The output of the calibration procedure
is gain and offset. The gain is used by the software while
the offset is programmed in a trim register. These trim values are
offset by bms every time it does an auto vsense read.

Change-Id: I20f6a7173321aa68967803d67c9a4813bcd75fae
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>