ppc4xx: Remove superfluous dram_init() call or replace it by initdram()

Historically the 405 U-Boot port had a dram_init() call in early init
stage. This function was still called from start.S and most of the time
coded in assembler. This is not needed anymore (since a long time) and
boards should implement the common initdram() function in C instead.

This patch now removed the dram_init() call from start.S and removes the
empty implementations that are scattered through most of the 405 board
ports. Some older board ports really implement this dram_init() though.
These are:

csb272
csb472
ERIC
EXBITGEN
W7OLMC
W7OLMG

I changed those boards to call this assembler dram_init() function now
from their board specific initdram() instead. This *should* work, but please
test again on those platforms. And it is perhaps a good idea that those
boards use some common 405 SDRAM initialization code from cpu/ppc4xx at
some time. So further patches welcome here.

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