DT: add binding documentation for STTS751

* 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>;

}