dt-bindings: drm/msm/gpu: Add cooling device support

Add cooling device support to gpu. A cooling device is bound to a
thermal zone to allow thermal mitigation.
Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>

Documentation/devicetree/bindings/display/msm/gpu.txt

@@ -39,6 +39,10 @@ Required properties:
         a4xx Snapdragon SoCs. See
         Documentation/devicetree/bindings/sram/qcom,ocmem.yaml.
 
+Optional properties:
+- #cooling-cells: The value must be 2. For details, please refer
+	Documentation/devicetree/bindings/thermal/thermal-cooling-devices.yaml.
+
 Example 3xx/4xx:
 
 / {
@@ -61,6 +65,7 @@ Example 3xx/4xx:
 		power-domains = <&mmcc OXILICX_GDSC>;
 		operating-points-v2 = <&gpu_opp_table>;
 		iommus = <&gpu_iommu 0>;
+		#cooling-cells = <2>;
 	};
 
 	gpu_sram: ocmem@fdd00000 {
@@ -98,6 +103,8 @@ Example a6xx (with GMU):
 		reg = <0x5000000 0x40000>, <0x509e000 0x10>;
 		reg-names = "kgsl_3d0_reg_memory", "cx_mem";
 
+		#cooling-cells = <2>;
+
 		/*
 		 * Look ma, no clocks! The GPU clocks and power are
 		 * controlled entirely by the GMU