thermal: rcar_gen3_thermal: Document the R-Car Gen3

* DT bindings for Renesas R-Car Gen3 Thermal Sensor driver

On R-Car Gen3 SoCs, the thermal sensor controllers (TSC) control the thermal

sensors (THS) which are the analog circuits for measuring temperature (Tj)

inside the LSI.

Required properties:

- compatible		: "renesas,<soctype>-thermal",

			  Examples with soctypes are:

			    - "renesas,r8a7795-thermal" (R-Car H3)

			    - "renesas,r8a7796-thermal" (R-Car M3-W)

- reg			: Address ranges of the thermal registers. Each sensor

			  needs one address range. Sorting must be done in

			  increasing order according to datasheet, i.e.

			  TSC1, TSC2, ...

- clocks		: Must contain a reference to the functional clock.

- #thermal-sensor-cells : must be <1>.

Optional properties:

- interrupts           : interrupts routed to the TSC (3 for H3 and M3-W)

- power-domain		: Must contain a reference to the power domain. This

			  property is mandatory if the thermal sensor instance

			  is part of a controllable power domain.

Example:

	tsc: thermal@e6198000 {

		compatible = "renesas,r8a7795-thermal";

		reg = <0 0xe6198000 0 0x68>,

		      <0 0xe61a0000 0 0x5c>,

		      <0 0xe61a8000 0 0x5c>;

		interrupts = <GIC_SPI 67 IRQ_TYPE_LEVEL_HIGH>,

			     <GIC_SPI 68 IRQ_TYPE_LEVEL_HIGH>,

			     <GIC_SPI 69 IRQ_TYPE_LEVEL_HIGH>;

		clocks = <&cpg CPG_MOD 522>;

		power-domains = <&sysc R8A7795_PD_ALWAYS_ON>;

		#thermal-sensor-cells = <1>;

		status = "okay";

	};

	thermal-zones {

		sensor_thermal1: sensor-thermal1 {

			polling-delay-passive = <250>;

			polling-delay = <1000>;

			thermal-sensors = <&tsc 0>;

			trips {

				sensor1_crit: sensor1-crit {

					temperature = <90000>;

					hysteresis = <2000>;

					type = "critical";

				};

			};

		};

	};