dt-bindings: thermal: al-thermal: Add binding documentation

Amazon's Annapurna Labs Thermal Sensor

Simple thermal device that allows temperature reading by a single MMIO

transaction.

Required properties:

- compatible: "amazon,al-thermal".

- reg: The physical base address and length of the sensor's registers.

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

Example:

	thermal: thermal {

		compatible = "amazon,al-thermal";

		reg = <0x0 0x05002860 0x0 0x1>;

		#thermal-sensor-cells = <0x1>;

	};

	thermal-zones {

		thermal-z0 {

			polling-delay-passive = <250>;

			polling-delay = <1000>;

			thermal-sensors = <&thermal 0>;

			trips {

				critical {

					temperature = <105000>;

					hysteresis = <2000>;

					type = "critical";

				};

			};

		};

	};