power: pm8921-bms: add api to simultaneously read vbatt and ibatt

The compensation algorithms need simultaneous readings of the battery
voltage (vbatt) and the current flowing through it (ibatt).

One needs to put the BMS in override mode to get simultaneous vbatt
and ibatt readings.

Note that since one would need to sleep for 20 milliseconds after the
BMS is put in override mode the spin_lock is changed to mutex in the bms
driver.

Change-Id: I1bfdc1f4e84d5d3c2cd6502dd66afdfba650cc0d
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>