dt-bindings: iio: dac: Add ti,dac7311 device tree bindings in documentation

Description how to invoke ti-dac7311 driver from device tree file.
Signed-off-by: Charles-Antoine Couret <charles-antoine.couret@essensium.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/dac/ti,dac7311.txt

+TI DAC7311 device tree bindings
+
+Required properties:
+- compatible: must be set to:
+	* "ti,dac7311"
+	* "ti,dac6311"
+	* "ti,dac5311"
+- reg: spi chip select number for the device
+- vref-supply: The regulator supply for ADC reference voltage
+
+Optional properties:
+- spi-max-frequency: Max SPI frequency to use
+
+Example:
+
+	spi_master {
+		dac@0 {
+			compatible = "ti,dac7311";
+			reg = <0>; /* CS0 */
+			spi-max-frequency = <1000000>;
+			vref-supply = <&vdd_supply>;
+		};
+	};