arm: mvebu: enable the SDIO interface on the Globalscale Mirabox

The Globalscale Mirabox uses the SDIO interface of the Armada 370 to
connect to a Wifi/Bluetooth SD8787 chip, so we enable the SDIO
interface of this board in its Device Tree file.

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