ppc4xx: Cleanup for partial linking and --gc-sections

This commit adapts 4xx boards for partial linking with --gc-sections.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Tirumala Marri <tmarri@apm.com>
Cc: David Updegraff <dave@cray.com>
Cc: Matthias Fuchs <matthias.fuchs@esd-electronics.com>
Cc: Dirk Eibach <eibach@gdsys.de>
Cc: Larry Johnson <lrj@acm.org>
Cc: Peter De Schrijver <p2@mind.be>
Cc: Niklaus Giger <niklaus.giger@netstal.com>
Cc: Daniel Poirot <dan.poirot@windriver.com>
Acked-by: Stefan Roese <sr@denx.de>