dt-bindings: iio: chemical: pms7003: add device tree support

Add device tree support for Plantower PMS7003 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/plantower,pms7003.txt

+* Plantower PMS7003 particulate matter sensor
+
+Required properties:
+- compatible: must be "plantower,pms7003"
+- vcc-supply: phandle to the regulator that provides power to the sensor
+
+Optional properties:
+- plantower,set-gpios: phandle to the GPIO connected to the SET line
+- reset-gpios: phandle to the GPIO connected to the RESET line
+
+Refer to serial/slave-device.txt for generic serial attached device bindings.
+
+Example:
+
+&uart0 {
+	air-pollution-sensor {
+		compatible = "plantower,pms7003";
+		vcc-supply = <&reg_vcc5v0>;
+	};
+};