power: pm8921-bms: periodic hkadc calibration

Introduce periodic hkadc calibration again. A bug was introduced when
periodic ccadc calibration code was moved to the ccadc driver - hkadc
calibration got dropped.

Reintroduce periodic hkadc calibration.

Change-Id: Ib22e03ce881320ca556bfbddd55e658b42363065
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>