MAINTAINERS: Add myself as co-maintainer to the legacy support of the mvebu SoCs

I will also take care of the legacy support(not fully converted to DT)
of the mvebu SoCs.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>