iio: potentiometer: add a driver for Maxim 5432-5435

Add a driver for the Maxim Integrated MAX5432-MAX5435 family of digital
potentiometers.

These potentiometers are connected via I2C and have 32 wiper
positions.

Supported functionality
- set the volatile wiper position
- read the potentiometer scale

Datasheet:
https://datasheets.maximintegrated.com/en/ds/MAX5432-MAX5435.pdf



Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>