arm: mvebu: enable the SD card slot on Armada 370 DB board

arm: mvebu: enable the SD card slot on Armada 370 DB board

The Armada XP DB evaluation board has one SD card slot, directly
connected to the SDIO IP of the SoC, so we add a device tree
description for it.

However, in the default configuration of the board, the SD card slot
is not usable: the connector plugged into CON40 must be changed
against a different one, provided with the board by the
manufacturer. Since such a manual modification of the hardware is
needed, we did not enable the SDIO interface by default, and left it
to the board user to modify the Device Tree if needed. Since this
board is really only an evaluation board for developers and not a
final product, it is not too bad.

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