mmc: mxs-mmc: copy wp_gpio in struct mxs_mmc_host

Copy wp_gpio from platform_data into struct mxs_mmc_host, so that
the use of platform_data can be limited in probe function, which
will ease the device tree probe.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Chris Ball <cjb@laptop.org>