arm: mvebu: enable the SD card slot on Armada XP DB board

The Armada XP DB evaluation board has one SD card slot, directly
connected to the SDIO IP of the SoC, so we enable this
IP. Unfortunately, there are no GPIOs for card-detect and
write-protect.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>