MAINTAINERS: Add device tree bindings to mv88e6xx section

Also include the netdev list for convenience, as done elsewhere.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Cc: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>