ARM: dts: add initial dts file for EXYNOS5250, SMDK5250

This patch adds initial dts file for EXYNOS5250 SoC. This dts
file is including the SoC specific devices and properties. And
adds the dts file for SMDK5250 board which uses the EXYNOS5250
dts file. Its board specific properites will be added later.

Cc: Thomas Abraham <thomas.abraham@linaro.org>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>