ARM: dts: igep0020: Add SD card write-protect pin.

A host device that supports write protection should refuse to write to
an SD card that is designated read-only when write-protect is set. This
is an optional feature of the SD specification.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>