ARM: mvebu: Update defconfig to select SPI flash and MTD support

The Armada XP DB-MV784MP-GP board has an SPI flash device.
These options allow to access that device over MTD.

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