hwmon: pm8921-adc: Add reverse calibration for BTM

-Change BTM threshold table to map temperature vs
input voltage instead of temperature vs A/D word.
Apply reverse calibration to convert the input
voltage to A/D word.
-Fix representing low/high threshold voltage as
a signed int.

CRs-Fixed: 313653
Change-Id: Ib565c672130f8a246a05636c1353ef0d5b373b7d
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>