exynos5: config: Rename the smdk5250.h to exynos5250-dt.h

exynos5: config: Rename the smdk5250.h to exynos5250-dt.h

Create a common configuration file for all exynos5250 based boards.
Going forward we will be using DT based driver discovery for all the boards
based on Exynos5. The different boards added will have there own config.h files
which internally will include this file and specify their specific DT files.

Signed-off-by: Hatim Ali <hatim.rv@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>