iio: potentiometer: add driver for Maxim Integrated DS1803

The following functions are supported:
 - write, read potentiometer value
 - potentiometer scale

Datasheet: https://datasheets.maximintegrated.com/en/ds/DS1803.pdf

Signed-off-by: Slawomir Stepien <sst@poczta.fm>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>