iio: mcp3422: Add DT binding documentation

Adding binding documentation for ADC MCP3422.
Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/adc/mcp3422.txt

+* Microchip mcp3422/3/4/6/7/8 chip family (ADC)
+
+Required properties:
+ - compatible: Should be
+	"microchip,mcp3422" or
+	"microchip,mcp3423" or
+	"microchip,mcp3424" or
+	"microchip,mcp3426" or
+	"microchip,mcp3427" or
+	"microchip,mcp3428"
+ - reg: I2C address for the device
+
+Example:
+adc@0 {
+	compatible = "microchip,mcp3424";
+	reg = <0x68>;
+};