iio: adc: qcom-vadc-common: add reverse scaling for PMIC5 Gen2 ADC_TM

Add reverse scaling function for PMIC5 Gen2 ADC_TM, to convert
temperature to raw ADC code, for setting thresholds for
thermistor channels.

Signed-off-by: Jishnu Prakash <quic_jprakash@quicinc.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/1648991869-20899-3-git-send-email-quic_jprakash@quicinc.com


Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>