ARM: dts: Add disable-wp for sd card slot on smdk5250

The next change will remove the code from the dw_mmc-exynos that added
the DW_MCI_QUIRK_NO_WRITE_PROTECT.  Keep existing functionality of
having no write protect pin on smdk5250 by adding the disable-wp
property.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Chris Ball <cjb@laptop.org>