ARM: dts: imx6: RIoTboard Add chosen stdout-path property

The RIoTboard debug uart is connected to serial1.
Add a chosen property in the DTS so OS knows what serial port to use for
the console.

Signed-off-by: Emmanuel Vadot <manu@freebsd.org>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>