ARM: dts: imx: Pass a label to the soc node

Pass a label to the 'soc' node to make it easier to reference
it from other devicetree files.

U-Boot, for example usually needs to access the AIPS node to
pass U-Boot-specific properties.

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