ppc4xx: Big header cleanup, mostly PPC440 related

This patch starts a bit PPC4xx header cleanup. First patch mostly
touches PPC440 files. A later patch will touch the PPC405 files as well.

This cleanup is done by creating header files for all SoC versions and
moving the SoC specific defines into these special headers. This way the
common header ppc405.h and ppc440.h can be cleaned up finally.

Signed-off-by: Stefan Roese <sr@denx.de>