staging: iio: ad5933: add binding doc for ad5933

Analog Devices AD5933/AD5934 Impedance Converter, Network Analyzer

https://www.analog.com/media/en/technical-documentation/data-sheets/AD5933.pdf

https://www.analog.com/media/en/technical-documentation/data-sheets/AD5934.pdf

Required properties:

 - compatible : should be one of

		"adi,ad5933"

		"adi,ad5934"

 - reg : the I2C address.

 - vdd-supply : The regulator supply for DVDD, AVDD1 and AVDD2 when they

   are connected together.

Optional properties:

- clocks : external clock reference.

- clock-names : must be "mclk" if clocks is set.

Example for a I2C device node:

	impedance-analyzer@0d {

		compatible = "adi,adxl345";

		reg = <0x0d>;

		vdd-supply = <&vdd_supply>;

		clocks = <&ref_clk>;

		clock-names = "mclk";

	};