iio: mcp4531: Driver for Microchip digital potentiometers

Add support for Microchip digital potentiometers and rheostats
	MCP4531, MCP4532, MCP4551, MCP4552
	MCP4631, MCP4632, MCP4651, MCP4652

DEVICE   Wipers  Steps  Resistor Opts (kOhm)  i2c address
MCP4531  1       129    5, 10, 50, 100        010111x
MCP4532  1       129    5, 10, 50, 100        01011xx
MCP4551  1       257    5, 10, 50, 100        010111x
MCP4552  1       257    5, 10, 50, 100        01011xx
MCP4631  2       129    5, 10, 50, 100        0101xxx
MCP4632  2       129    5, 10, 50, 100        01011xx
MCP4651  2       257    5, 10, 50, 100        0101xxx
MCP4652  2       257    5, 10, 50, 100        01011xx

Datasheet: http://www.microchip.com/downloads/en/DeviceDoc/22096b.pdf

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
diff --git a/drivers/iio/Makefile b/drivers/iio/Makefile
index e210055..aeca726 100644
--- a/drivers/iio/Makefile
+++ b/drivers/iio/Makefile
@@ -23,6 +23,7 @@
 obj-y += light/
 obj-y += magnetometer/
 obj-y += orientation/
+obj-y += potentiometer/
 obj-y += pressure/
 obj-y += proximity/
 obj-y += temperature/