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

Documentation/devicetree/bindings/iio/potentiometer/mcp4131.txt

+* Microchip MCP413X/414X/415X/416X/423X/424X/425X/426X Digital Potentiometer
+  driver
+
+The node for this driver must be a child node of a SPI controller, hence
+all mandatory properties described in
+
+        Documentation/devicetree/bindings/spi/spi-bus.txt
+
+must be specified.
+
+Required properties:
+	- compatible:  	Must be one of the following, depending on the
+			model:
+			"microchip,mcp4131-502"
+			"microchip,mcp4131-103"
+			"microchip,mcp4131-503"
+			"microchip,mcp4131-104"
+			"microchip,mcp4132-502"
+			"microchip,mcp4132-103"
+			"microchip,mcp4132-503"
+			"microchip,mcp4132-104"
+			"microchip,mcp4141-502"
+			"microchip,mcp4141-103"
+			"microchip,mcp4141-503"
+			"microchip,mcp4141-104"
+			"microchip,mcp4142-502"
+			"microchip,mcp4142-103"
+			"microchip,mcp4142-503"
+			"microchip,mcp4142-104"
+			"microchip,mcp4151-502"
+			"microchip,mcp4151-103"
+			"microchip,mcp4151-503"
+			"microchip,mcp4151-104"
+			"microchip,mcp4152-502"
+			"microchip,mcp4152-103"
+			"microchip,mcp4152-503"
+			"microchip,mcp4152-104"
+			"microchip,mcp4161-502"
+			"microchip,mcp4161-103"
+			"microchip,mcp4161-503"
+			"microchip,mcp4161-104"
+			"microchip,mcp4162-502"
+			"microchip,mcp4162-103"
+			"microchip,mcp4162-503"
+			"microchip,mcp4162-104"
+			"microchip,mcp4231-502"
+			"microchip,mcp4231-103"
+			"microchip,mcp4231-503"
+			"microchip,mcp4231-104"
+			"microchip,mcp4232-502"
+			"microchip,mcp4232-103"
+			"microchip,mcp4232-503"
+			"microchip,mcp4232-104"
+			"microchip,mcp4241-502"
+			"microchip,mcp4241-103"
+			"microchip,mcp4241-503"
+			"microchip,mcp4241-104"
+			"microchip,mcp4242-502"
+			"microchip,mcp4242-103"
+			"microchip,mcp4242-503"
+			"microchip,mcp4242-104"
+			"microchip,mcp4251-502"
+			"microchip,mcp4251-103"
+			"microchip,mcp4251-503"
+			"microchip,mcp4251-104"
+			"microchip,mcp4252-502"
+			"microchip,mcp4252-103"
+			"microchip,mcp4252-503"
+			"microchip,mcp4252-104"
+			"microchip,mcp4261-502"
+			"microchip,mcp4261-103"
+			"microchip,mcp4261-503"
+			"microchip,mcp4261-104"
+			"microchip,mcp4262-502"
+			"microchip,mcp4262-103"
+			"microchip,mcp4262-503"
+			"microchip,mcp4262-104"
+
+Example:
+mcp4131: mcp4131@0 {
+	compatible = "mcp4131-502";
+	reg = <0>;
+	spi-max-frequency = <500000>;
+};

drivers/iio/potentiometer/Kconfig

@@ -5,6 +5,24 @@
 
 menu "Digital potentiometers"
 
+config MCP4131
+	tristate "Microchip MCP413X/414X/415X/416X/423X/424X/425X/426X Digital Potentiometer driver"
+	depends on SPI
+	help
+	  Say yes here to build support for the Microchip
+	  MCP4131, MCP4132,
+	  MCP4141, MCP4142,
+	  MCP4151, MCP4152,
+	  MCP4161, MCP4162,
+	  MCP4231, MCP4232,
+	  MCP4241, MCP4242,
+	  MCP4251, MCP4252,
+	  MCP4261, MCP4262,
+	  digital potentiomenter chips.
+
+	  To compile this driver as a module, choose M here: the
+	  module will be called mcp4131.
+
 config MCP4531
 	tristate "Microchip MCP45xx/MCP46xx Digital Potentiometer driver"
 	depends on I2C

drivers/iio/potentiometer/Makefile

@@ -3,5 +3,6 @@
 #
 
 # When adding new entries keep the list in alphabetical order
+obj-$(CONFIG_MCP4131) += mcp4131.o
 obj-$(CONFIG_MCP4531) += mcp4531.o
 obj-$(CONFIG_TPL0102) += tpl0102.o