iio: adc: Add Texas Instruments ADC081C021/027 support

Add support for reading conversion results from the ADC and provide them
through a single IIO channel. A proper scaling factor is also exported
based on the reference voltage provided by a regulator.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>