On some SoCs (like exynos5250) you need to use an external GPIO for
write protect.  Add support for wp-gpios to the core dw_mmc driver
since it could be useful across multiple SoCs.

With this change I am able to make use of the write protect for the
external SD slot on exynos5250-snow.
Signed-off-by: NDoug Anderson <dianders@chromium.org>
Acked-by: NSeungwon Jeon <tgih.jun@samsung.com>
Acked-by: NOlof Johansson <olof@lixom.net>
Signed-off-by: NChris Ball <cjb@laptop.org>