MAINTAINERS: Update Marvell custodianship

Add myself as custodian for the Marvell git repository. Additionally,
add the mach-mvebu directory to the list of files / directories. And
add Armada XP & Armada 38x to the title (not only kirkwood).

Signed-off-by: Stefan Roese <sr@denx.de>
Acked-by: Tom Rini <trini@konsulko.com>
Acked-by: Prafulla Wadaskar <prafulla@marvell.com>
Acked-by: Luka Perkov <luka.perkov@sartura.hr>