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