iio:adc:vf610-adc: Add temperature sensor support

Vybrid ADC peripheral includes a temperature sensor
which is connected to channel number 26. This patch
adds support for the sensor. The raw value is read
and the temperature calculated in milli degree Celsius,
which is returned using IIO_CHAN_INFO_PROCESSED option.

Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>