dt-bindings: ap806: document the thermal interrupt capabilities

The thermal IP can probe the temperature all around the processor. It

may feature several channels, each of them wired to one sensor.

It is possible to setup an overheat interrupt by giving at least one

critical point to any subnode of the thermal-zone node.

Required properties:

- compatible: must be one of:

  * marvell,armada-ap806-thermal

- reg: register range associated with the thermal functions.

Optional properties:

- interrupts: overheat interrupt handle. Should point to line 18 of the

  SEI irqchip. See interrupt-controller/interrupts.txt

- #thermal-sensor-cells: shall be <1> when thermal-zones subnodes refer

  to this IP and represents the channel ID. There is one sensor per

  channel. O refers to the thermal IP internal channel, while positive

	ap_thermal: thermal-sensor@80 {

		compatible = "marvell,armada-ap806-thermal";

		reg = <0x80 0x10>;

		interrupt-parent = <&sei>;

		interrupts = <18>;

		#thermal-sensor-cells = <1>;

	};

};