ARM: dts: imx6qdl-icore: Add missing stdout-path property

This would help us to get early boot logs by passing "earlycon" to
kernel bootargs.

Further, by adding this we don't have to depend on complex earlyprintk
configs for early boot logs.

Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shyam Saini <shyam.saini@amarulasolutions.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>