power: pm8921-bms: api for battery insertion/removal

Provide api to clean up the history values the driver remembers to
report the soc. These apis will be called when the battery is removed.
Similarly provide api to initiate battery SOC calculations upon battery
insertion.

Change-Id: Ie9f3f5f45c789efc2ba6d39226401691912c764c
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>