iio: potentiometer: add driver for Microchip MCP413X/414X/415X/416X/423X/424X/425X/426X

The following functionalities are supported:
 - write, read from volatile memory

Datasheet: http://ww1.microchip.com/downloads/en/DeviceDoc/22060b.pdf



Signed-off-by: Slawomir Stepien <sst@poczta.fm>
Reviewed-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>