hwmon: qpnp-adc: Add scaling support for PMI CHG channel

The voltage measured by the HKADC is related to the junction
temperature and scaled by the following equation :-
tj = -138.89degC * (v_adc * 2) + 391.75degC

Change-Id: Ibd6185c3926f9f5e482b3990adff3eee61e7caa5
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>