MAINTAINERS: add arcnet and take maintainership

Add entry for arcnet to MAINTAINERS file and add myself as the
maintainer of the subsystem.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Cc: davem@davemloft.net
Cc: joe@perches.com
Signed-off-by: David S. Miller <davem@davemloft.net>