dt-bindings: ad5272: Add bindings for Analog Devices digital potentiometers

* Analog Devices AD5272 digital potentiometer

The node for this device must be a child node of a I2C controller, hence

all mandatory properties for your controller must be specified. See directory:

        Documentation/devicetree/bindings/i2c

for more details.

Required properties:

	- compatible:  	Must be one of the following, depending on the model:

			adi,ad5272-020

			adi,ad5272-050

			adi,ad5272-100

			adi,ad5274-020

			adi,ad5274-100

Optional properties:

 - reset-gpios: GPIO specification for the RESET input. This is an

		active low signal to the AD5272.

Example:

ad5272: potentiometer@2f {

	reg = <0x2F>;

	compatible = "adi,ad5272-020";

	reset-gpios = <&gpio3 6 GPIO_ACTIVE_HIGH>;

};