arm: mvebu: Remove unreferenced define

MAX_MVNETA_DEVS is not used anywhere in U-Boot. So lets remove it.

Signed-off-by: Stefan Roese <sr@denx.de>
Tested-by: Kevin Smith <kevin.smith@elecsyscorp.com>
Tested-by: Dirk Eibach <dirk.eibach@gdsys.cc>