ARM: dts: Add initial support for Arndale Octa board

Arndale Octa board is based on Exynos5420 SoC. This patch
adds the basic support required for booting it through DT.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>