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

* 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>;

	};

};