Move thermal-zone node from the soc node to the root node.

thermal-zone node does not have any register properties and thus
shouldn't be placed on the bus.

This fixes the following build warnings with W=1:

arch/arm64/boot/dts/freescale/fsl-ls1028a.dtsi:583.17-612.5: Warning (simple_bus_reg): /soc/thermal-zones: missing or empty reg/ranges property
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>