ARM: dts: imx6qdl-wandboard: Add stdout-path

Setting a stdout-path in the .dtb is convenient because then the user
gets a serial console on the RS-232 connector without any extra effort
of figuring out the relevant 'console=' boot parameter.

Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>