dt-bindings: thermal: Add documentation for CPU voltage cooling device

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CPU Voltage cooling device:

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A single cluster can have CPUs with different frequency plans.

Mitigating individual CPU may or may not bring down the CPU cluster

voltage depending on the operating level of the other CPUs.

CPU voltage cooling device will provide support to apply CPU frequency

mitigation on the different CPUs in a cluster to achieve a reduction in

cluster voltage. This is achieved by building a mitigation table mapping

the CPU frequency levels to a voltage.

Required Parameters:

- compatible: must be 'qti,cc-cooling-devices' for cpu voltage cooling device

  driver.

	voltage cooling device node properties:

	-qcom,cpus:

		Usage: required

		Value type: An array of CPU phandle

		Definition: Specify array of 2 CPU phandles, which needs to be

			used for building a voltage based mitigation table.

	-#cooling-cells:

		Usage: required

		Value type: <integer>

		Definition: Must be 2. Needed for of_thermal as cooling device

			identifier. Please refer to

			<devicetree/bindings/thermal/thermal.txt> for more

			details.

Optional Parameters:

Example:

	qcom,cpu-voltage-cdev {

		compatible = "qti,cc-cooling-devices";

		apc1_cluster: qcom,apc1-cluster {

			qcom,cpus = <&CPU4 &CPU7>;

			#cooling-cells = <2>;

		};

	};