ARM: mvebu: enable UBI and UBIFS in mvebu_v7_defconfig

UBIFS is commonly used on Marvell EBU v7 platforms to store the root
file-system, so it makes sense to have those options enabled by default
in mvebu_v7_defconfig.

Signed-off-by: Grzegorz Jaszczyk <jaz@semihalf.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>