arm: mvebu: Only define MV88F78X60 for Armada XP

This define is used by the DDR training code for Armada XP. With the
upcoming addition of Armada 38x support, lets only define it for
Armada XP in this common header.

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