dt-bindings: Document the hi6220 thermal sensor bindings

* Temperature Sensor on hisilicon SoCs

** Required properties :

- compatible: "hisilicon,tsensor".

- reg: physical base address of thermal sensor and length of memory mapped

  region.

- interrupt: The interrupt number to the cpu. Defines the interrupt used

  by /SOCTHERM/tsensor.

- clock-names: Input clock name, should be 'thermal_clk'.

- clocks: phandles for clock specified in "clock-names" property.

- #thermal-sensor-cells: Should be 1. See ./thermal.txt for a description.

Example :

	tsensor: tsensor@0,f7030700 {

		compatible = "hisilicon,tsensor";

		reg = <0x0 0xf7030700 0x0 0x1000>;

		interrupts = <0 7 0x4>;

		clocks = <&sys_ctrl HI6220_TSENSOR_CLK>;

		clock-names = "thermal_clk";

		#thermal-sensor-cells = <1>;

	}