iio:dac: Add DT binding documentation for ds4424

Signed-off-by: Ismail H. Kose <ihkose@gmail.com>
Signed-off-by: Ismail H. Kose <Ismail.Kose@maximintegrated.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/dac/ds4424.txt

+Maxim Integrated DS4422/DS4424 7-bit Sink/Source Current DAC Device Driver
+
+Datasheet publicly available at:
+https://datasheets.maximintegrated.com/en/ds/DS4422-DS4424.pdf
+
+Required properties:
+	- compatible: Should be one of
+		maxim,ds4422
+		maxim,ds4424
+	- reg: Should contain the DAC I2C address
+
+Optional properties:
+	- vcc-supply: Power supply is optional. If not defined, driver will ignore it.
+
+Example:
+	ds4224@10 {
+		compatible = "maxim,ds4424";
+		reg = <0x10>; /* When A0, A1 pins are ground */
+		vcc-supply = <&vcc_3v3>;
+	};