arm: mvebu: Change header macros from ARMADA_XP to MVEBU

Since these files will be used for other MVEBU SoC's, lets reflect this
in the headers marcos as well.

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