hwmon: qpnp-adc: Add reverse scaling API

Add separate VADC reverse scaling function to consider
the channel properties that include the pre-div scaling
for the reverse calibration on VADC channels to support
recurring channel measurement mode.

Change-Id: Ifd863c3c3fd0c3c7f93b80fac8cf186f10c69e38
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>