MAINTAINERS: add myself as Marvell berlin SoC maintainer

I would like to take maintainership for Marvell berlin SoCs.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Olof Johansson <olof@lixom.net>