ARM: dts: imx6-phytec: Use the standard 'stdout-path' property

Use the standard 'stdout-path' property to fix the following DTC warnings:

arch/arm/boot/dts/imx6dl-phytec-mira-rdk-nand.dtb: Warning
(chosen_node_stdout_path): /chosen:linux,stdout-path: Use
'stdout-path' instead

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>