ARM: dts: Fill in bootargs for exynos5250-snow

exynos5250-cros-common.dtsi had an empty /chosen node.
Fill in exemplary boot arguments.

Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Andreas Faerber <afaerber@suse.de>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>