staging: iio: ad5933: add binding doc for ad5933

Add a devicetree documentation for the ad5933 and ad5934 impedance
converter, network analyzer.
Signed-off-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Signed-off-by: Gabriel Capella <gabriel@capella.pro>
Co-Developed-by: Gabriel Capella <gabriel@capella.pro>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/impedance-analyzer/ad5933.txt

+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";
+	};