ARM: dts: imx: Add an cpu0 label for imx6dl devices

Adding the label cpu0 allows the adjustment of cpu-parameters
by reference in overlaying dtsi files in the same way as it
is possible for imx6q devices.

Signed-off-by: Jan Tuerk <jan.tuerk@emtrion.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>