arm: mvebu: enable SDIO support in mvebu_defconfig

Now that the mvsdio driver has gained Device Tree support and the
necessary Device Tree informations has been added for Armada 370 and
Armada XP platforms, we enable the MMC subsystem and the mvsdio driver
in mvebu_defconfig.

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