power: pm8921-bms: read temperature and voltage via adc

Read the battery temeperature and voltage from the adc driver instead
of hard coding them in the driver.

In the charing began and charing end functions, use
pm8921_bms_get_percent_charge api. This prevents duplicating calling
the adc functions.

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