ARM: dts: Fix MMC pinctrl for exynos5250-snow

The pinctrl properties should be on the device directly and not on the
slot sub-node.

Reported-by: Doug Anderson <dianders@chromium.org>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Andreas Faerber <afaerber@suse.de>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>