dt-bindings: hwmon: add dps650ab documentation

Bindings for Delta Electronics DPS-650-AB power supply.
Signed-off-by: Xiaoting Liu <xiaoting.liu@hxt-semitech.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/devicetree/bindings/hwmon/dps650ab.txt

+Bindings for Delta Electronics DPS-650-AB power supply
+
+Required properties:
+- compatible : "delta,dps650ab"
+- reg        : I2C address, one of 0x58, 0x59.
+
+Example:
+	dps650ab@58 {
+		    compatible = "delta,dps650ab";
+		    reg = <0x58>;
+	};