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

Add documentation for CPU voltage cooling device. 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.

Change-Id: I41a222c54442bdc065267f1f1cf079118b8032e9

bindings/thermal/qti-voltage-cooling.txt

@@ -0,0 +1,42 @@
+===============================================================================
+CPU Voltage cooling device:
+===============================================================================
+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>;
+		};
+	};