Move disp0 node from soc node to root node.

disp0 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/arm/boot/dts/imx6dl-aristainetos_4.dtb: Warning (simple_bus_reg): Node /soc/disp0 missing or empty reg/ranges property
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Tested-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>