ARM: dts: Move the dwmmc aliases from smdk5250 dts to exynos

The aliases for dwmmc were placed in the SMDK5250 dts file but really
should be common for all exynos5250 boards.  Move it to the common CPU
file.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>