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