power: pm8921-bms: expose coulomb counter based charge

Average current drawn can be calculated by reading the coulomb
counter based charge before and after the usecase is run and
dividing the difference in the charge by the time it took to
run the usecase.

Use power supply property POWER_SUPPLY_PROP_CHARGE_NOW, to
expose the coulomb counter based charge.

Change-Id: I43e26a2932ab3e3d9d79bb5af7daf2364ca133b7
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>