hwmon: qpnp-adc-voltage: Add temperature compensation

The VADC does not meet VBAT and OCV accuracy spec with just
gain and offset calibration. Additional compensation is required
on top of gain/offset calibrated reading to compensate for
variations based on chip revision, manufacturer and tempreature.

Change-Id: I9e08987ac39ac8de8e880b96bee4f8bc76a88e46
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>