MAINTAINERS: attach arch/arm/configs/mvebu_*_defconfig to relevant maintainers

The arch/arm/configs/mvebu_*_defconfig currently don't belong to any
maintainer, so get_maintainer returns only information based on
commits, which is not completely accurate.

Therefore, this commit replaces a useless empty newline in MAINTAINERS
by a line attaching those defconfigs to the appropriate Marvell EBU
maintainers.

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