thermal: qpnp-adc-tm: Add reverse scaling

Add reverse scaling to convert voltage in uV using absolute
calibration to raw ADC code written to set high/low threshold
for a specified BTM channel.

Change-Id: Ic56465df5ab60100cffe4303ff983480b71c0c95
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>