arm: mvebu: Add d-cache invalidate before enabling the d-cache

This solves some RX problems that have been seen, when using the
mvneta ethernet driver. The cache needs to be reset into a "clean"
state before using 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>