ARM: dts: ls1021a: move thermal-zones node out of soc/

This fixes dtbs-check error from simple-bus schema:
soc: thermal-zones: {'type': 'object'} is not allowed for {'cpu-thermal': ..... }
        From schema: /home/leo/.local/lib/python3.8/site-packages/dtschema/schemas/simple-bus.yaml

Signed-off-by: Li Yang <leoyang.li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>