dt-bindings: iio: chemical: sps30: add device tree support

Add device tree support for Sensirion SPS30 particulate
matter sensor.
Signed-off-by: Tomasz Duszynski <tduszyns@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/chemical/sensirion,sps30.txt

+* Sensirion SPS30 particulate matter sensor
+
+Required properties:
+- compatible: must be "sensirion,sps30"
+- reg: the I2C address of the sensor
+
+Example:
+
+sps30@69 {
+	compatible = "sensirion,sps30";
+	reg = <0x69>;
+};