iio: adc: Add Maxim max9611 ADC driver

Add iio driver for Maxim max9611 and max9612 current-sense amplifiers
with 12-bits ADC interface.

Datasheet publicly available at:
https://datasheets.maximintegrated.com/en/ds/MAX9611-MAX9612.pdf



Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>