power: pm8921-bms: calibrate xoadc more often

Improve the values of the battery voltage read from xoadc by periodically
calibrating the xoadc.

The xoadc needs to be calibrated every time a charger is plugged in
or plugged out. This is because the reference voltage shifts a bit
when a charger is plugged in.

Provide an api to calibrate xoadc. This api will be used by the charger
driver.

CRs-Fixed: 334825
Change-Id: I09735f6e5c5d4ab8be379fe001fff81c25c95397
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>