dt-bindings: Add DDR cooling device

QTI DDR cooling device.

The DDR cooling device will be used to place a DDR frequency vote. This

cooling device will be used in those cases where all the subsystem's are

thermally throttled and DDR has to be operated with a minimum performance

level. This cooling device vote can ensure the same.

Properties:

- compatible:

	Usage: required

	Value type: <string>

	Definition: should be "qcom,ddr-cooling-device"

Cooling device node:

- qcom,ddr-freq:

	Usage: required

	Value type: <u32 array>

	Definition: List of available DDR frequencies.

-#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.

Example:

	qcom,ddr-cdev {

		compatible = "qcom,ddr-cooling-device";

		qcom,ddr-freq = <200000>,

				<451000>,

				<547000>,

				<681000>,

				<768000>,

				<1017000>,

				<1555000>,

				<1708000>,

				<2092000>,

				<2736000>,

				<3196000>;

		#cooling-cells = <2>;

        };