thermal: generic-adc: Add DT binding for ADC based thermal sensor

Sometimes, thermal sensors like NCT thermistors are connected to
the ADC channel. The temperature is read by reading the voltage
across the sensor resistance via ADC and referring the lookup
table for ADC value to temperature.

Add DT binding doc for the ADC based thermal sensor driver to
detail the DT property and provide the example for how to use it.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Acked-by: Rob Herring <rob@kernel.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>