leds/tca6507: DT: Add documentation for tca6507 devicetree bindings.

LEDs conected to tca6507

Required properties:

- compatible : should be : "ti,tca6507".

Each led is represented as a sub-node of the ti,tca6507 device.

LED sub-node properties:

- label : (optional) see Documentation/devicetree/bindings/leds/common.txt

- reg : number of LED line (could be from 0 to 6)

- linux,default-trigger : (optional)

   see Documentation/devicetree/bindings/leds/common.txt

Examples:

tca6507@45 {

	compatible = "ti,tca6507";

	#address-cells = <1>;

	#size-cells = <0>;

	reg = <0x45>;

	led0: red-aux@0 {

		label = "red:aux";

		reg = <0x0>;

	};

	led1: green-aux@1 {

		label = "green:aux";

		reg = <0x5>;

		linux,default-trigger = "default-on";

	};

};