iio: potentiometer: mcp4018: driver for Microchip digital potentiometers

Add support for Microchip digital potentiometers and rheostats
	MCP4017, MCP4018, MCP4019

They all have one wiper with 128 steps and come in 5, 10, 50 and 100 kOhm
variations.

Datasheet: http://www.microchip.com/downloads/en/DeviceDoc/22147a.pdf



Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>