ppc: Make ppc4xx ready for CONFIG_SYS_GENERIC_BOARD

The generic board  infrastructure assumes that gd is set by
arch code.

Signed-off-by: Dirk Eibach <dirk.eibach@gdsys.cc>
Signed-off-by: Stefan Roese <sr@denx.de>
board: Add CONFIG_SYS_GENERIC_BOARD to all gdsys boards

Add the generic board infrastructure to all gdsys boards.

Signed-off-by: Dirk Eibach <dirk.eibach@gdsys.cc>
Signed-off-by: Stefan Roese <sr@denx.de>
Merge git://www.denx.de/git/u-boot-ppc4xx