MAINTAINERS: mv643xx_eth: take over maintainership from Lennart

Lennart says: "I haven't been able to spend time on mv643xx_eth for a
while now, so if you want to take over maintainership, I'd be fine with
that."

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Lennert Buytenhek <buytenh@wantstofly.org>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: David S. Miller <davem@davemloft.net>