hwmon: pm8xxx-adc: Merge pm8921-adc and pm8xxx-adc

Merge changes from pm8921-adc to pm8xxx-adc. The
changes are modification to the BTM lookup table
to map voltage vs temperature instead of A/D word
vs temperature and reverse calibration on the
input voltage read for BTM. Fix setting the
mpp_scale value to avoid compile error.

This keeps the pm8xxx-adc and pm8921-adc
functional code as the same except for the name
change allowing future patch to remove the
pm8921-adc driver.

Change-Id: I03a0b809cd232f7fe8dff86248651c9a2593be29
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>