ARM: dts: exynos: Use stdout path property on Arndale Octa board

Replacing bootargs with stdout-path property in chosen node allows using
early console by adding just 'earlycon' parameter to kernel command
line.

Suggested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>