ARM: mvebu: enable MTD command line partition table in mvebu_v7_defconfig

Marvell EBU v7 platforms contain various MTD devices, therefore it makes
sense to allow configuring the MTD partition tables via the kernel command
line.

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>