PPC4xx:HCU4/5 cleanup ecc/sdram init

Make ecc initialisation robust, as DDR2-ECC errors may be generated
while zeroing the RAM.

Return 16 bytes (a cacheline) less than the available memory, as the
board and/or PPC440EPx might have problems accessing the last bytes.

Signed-off-by: Niklaus Giger <niklaus.giger@netstal.com>