power: Add pm8921 battery monitoring driver

pm8921 chip is equipped with a smart battery gauge called bms.
BMS is capable of intelligently measuring battery parameters
under various loads, the software uses these reading to
accurately determine battery capacity.

Add driver for the bms module.

Change-Id: I0655c19bc9edc6aea15fb66b4de8d647a305416f
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>