ARM: dts: Define stdout-path property for exynos5250-snow

The kernel can use as the default console a serial port if is defined
as stdout device in the Device Tree.

This allows a board to be booted without the need of having a console
parameter in the kernel command line.

Currently the Snow DTS has a bootargs in the /chosen node and this is
kept since users that don't have a serial console on this board might
be using it to have the boot log shown in the display. This will have
more precedence than the stdout-path but it's fine since is only used
when CONFIG_ARM_ATAG_DTB_COMPAT_CMDLINE_EXTEND is enabled.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Kukjin Kim <kgene@kernel.org>