ARM: dts: exynos: Add serial path for Rinato board to get earlycon support

Add 'chosen' node with 'stdout-path' property to instruct kernel which
serial driver should be used for kernel console/logs. This allows to
enable earlycon messages by adding just 'earlycon' parameter to kernel
command line.

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