dt-bindings: gpu: add #cooling-cells property to the ARM Mali Midgard GPU binding

The GPU can be used as a thermal cooling device, add an optional
'#cooling-cells' property.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20190516172510.181473-1-mka@chromium.org

Documentation/devicetree/bindings/gpu/arm,mali-midgard.txt

@@ -37,6 +37,8 @@ Optional properties:
 - operating-points-v2 : Refer to Documentation/devicetree/bindings/opp/opp.txt
   for details.
 
+- #cooling-cells: Refer to Documentation/devicetree/bindings/thermal/thermal.txt
+  for details.
 
 Example for a Mali-T760:
 
@@ -51,6 +53,7 @@ gpu@ffa30000 {
 	mali-supply = <&vdd_gpu>;
 	operating-points-v2 = <&gpu_opp_table>;
 	power-domains = <&power RK3288_PD_GPU>;
+	#cooling-cells = <2>;
 };
 
 gpu_opp_table: opp_table0 {