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

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>