powerpc/ppc4xx: Use generic accessor functions for gdsys FPGA

A set of accessor functions was added to be able to access not only
memory mapped FPGA in a generic way.

Thanks to Wolfgang Denk for getting this sorted properly.

Signed-off-by: Dirk Eibach <dirk.eibach@gdsys.cc>
Signed-off-by: Stefan Roese <sr@denx.de>