MAINTAINERS: Add dts entries for some of the Marvell SoCs

Since many releases, the modifications of the mvebu and berlin device
tree files are merged through the mvebu subsystem. This patch makes it
official in order to help the contributors using the get_maintainer.pl
to find the accurate peoples.

In the same time, updated the mvebu description which now includes the
kirkwood SoCs and new Armada SoCs.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Acked-by: Andrew Lunn <andrew@lunn.ch>