ppc4xx: Consolidate PPC4xx SDRAM/DDR/DDR2 defines, part1

ppc4xx: Consolidate PPC4xx SDRAM/DDR/DDR2 defines, part1

This patch removes all SDRAM related defines from the PPC4xx headers
ppc405.h and ppc440.h. This is needed since now some 405 PPC's use
the same SDRAM controller as 440 systems do (like 405EX and 440SP).

It also introduces new defines for the equipped SDRAM controller based on
which PPC variant is used. There new defines are:

used on 405GR/CR/EP and some Xilinx Virtex boards.

used on 440GP/GX/EP/GR.

used on 440EPx/GRx.

used on 405EX/r/440SP/SPe/460EX/GT.

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