arm64: dts: imx8mq: Move thermal-zones out of bus node

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

Move thermal-zones node from soc node to root node in order to fix
the following build warning with W=1:

arch/arm64/boot/dts/freescale/imx8mq.dtsi:305.18-364.6: Warning (simple_bus_reg): /soc@0/bus@30000000/thermal-zones: missing or empty reg/ranges property

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>