arm: mvebu: Change network init code to allow a more flexible setup

With the introduction of the Armada 38x support, its necessary to change
the mvneta ethernet driver init call from always 4 times to a
configurable value. Lets make this init call more flexible by moving
the actually used devices to the config header.

Additionally this patch takes care of the slightly different base
addresses for the ethernet controllers on A38x.

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