iio: ti-dac5571: Add DT binding documentation

Adding binding documentation for Texas Instruments DAC5571 Family
Signed-off-by: Sean Nyekjaer <sean.nyekjaer@prevas.dk>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/dac/ti,dac5571.txt

+* Texas Instruments DAC5571 Family
+
+Required properties:
+ - compatible: Should contain
+    "ti,dac5571"
+    "ti,dac6571"
+    "ti,dac7571"
+    "ti,dac5574"
+    "ti,dac6574"
+    "ti,dac7574"
+    "ti,dac5573"
+    "ti,dac6573"
+    "ti,dac7573"
+ - reg: Should contain the DAC I2C address
+
+Optional properties:
+ - vref-supply: The regulator supply for DAC reference voltage
+
+Example:
+dac@0 {
+	compatible = "ti,dac5571";
+	reg = <0x4C>;
+	vref-supply = <&vdd_supply>;
+};