arm: mach-mvebu: add entry to MAINTAINERS

The new mach-mvebu directory will be maintained by Andrew Lunn and
Jason Cooper (as the existing maintainers for previous Marvell EBU
SoCs) together with Grégory Clément for the Armada 370/XP SoCs.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Lior Amsalem <alior@marvell.com>