mfd: pm8921: add bms cell

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 code to support the bms driver.

Change-Id: I5f14a82db0fda11adc0404ba58704a25fa7713af
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>