arm64: dts: add #cooling-cells to CPU nodes

BugLink: http://bugs.launchpad.net/bugs/1756866

commit acbf76ee upstream.

dtc complains about the lack of #coolin-cells properties for the
CPU nodes that are referred to as "cooling-device":

arch/arm64/boot/dts/mediatek/mt8173-evb.dtb: Warning (cooling_device_property): Missing property '#cooling-cells' in node /cpus/cpu@0 or bad phandle (referred from /thermal-zones/cpu_thermal/cooling-maps/map@0:cooling-device[0])
arch/arm64/boot/dts/mediatek/mt8173-evb.dtb: Warning (cooling_device_property): Missing property '#cooling-cells' in node /cpus/cpu@100 or bad phandle (referred from /thermal-zones/cpu_thermal/cooling-maps/map@1:cooling-device[0])

Apparently this property must be '<2>' to match the binding.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Olof Johansson <olof@lixom.net>
[arnd: backported to 4.15]
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

arch/arm64/boot/dts/mediatek/mt8173.dtsi

@@ -54,6 +54,7 @@ cpu0: cpu@0 {
 			reg = <0x000>;
 			enable-method = "psci";
 			cpu-idle-states = <&CPU_SLEEP_0>;
+			#cooling-cells = <2>;
 		};
 
 		cpu1: cpu@1 {
@@ -70,6 +71,7 @@ cpu2: cpu@100 {
 			reg = <0x100>;
 			enable-method = "psci";
 			cpu-idle-states = <&CPU_SLEEP_0>;
+			#cooling-cells = <2>;
 		};
 
 		cpu3: cpu@101 {