iio: adc: Add support for AD7091R5 ADC

AD7091R5 is 4-Channel, I2C, Ultra Low Power,12-Bit ADC.

This driver will also support AD7091R2/4/8 in the future.

Datasheet:
Link: https://www.analog.com/media/en/technical-documentation/data-sheets/ad7091r-5.pdf



Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Co-developed-by: Beniamin Bia <beniamin.bia@analog.com>
Signed-off-by: Beniamin Bia <beniamin.bia@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>