DT: add binding documentation for STTS751

Signed-off-by: NAndrea Merello <andrea.merello@gmail.com>
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>

Documentation/devicetree/bindings/hwmon/stts751.txt

+* STTS751 thermometer.
+
+Required node properties:
+- compatible: "stts751"
+- reg: I2C bus address of the device
+
+Optional properties:
+- smbus-timeout-disable: when set, the smbus timeout function will be disabled
+
+Example stts751 node:
+
+temp-sensor {
+	compatible = "stts751";
+	reg = <0x48>;
+}