ARM: mvebu: Add support for USB storage class in mvebu_defconfig

Some boards can have built-in USB storage class controllers so
it's better to have this option included by default.

Currently this option is needed to support built-in USB MMC controller
found in Globalscale Mirabox board.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>