arm: dts: i.mx8x: add #cooling-cells properties

Fix dtb building warnings:
Warning (cooling_device_property): /thermal-zones/cpu-thermal0/cooling-maps/map0:
Missing property '#cooling-cells' in node /cpus/cpu@0 or bad phandle (referred from cooling-device[0])

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Stefano Babic <sbabic@denx.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>