ppc: fix bamboo >256MB RAM initialization in hw/ppc4xx_devs.c

ppc: fix bamboo >256MB RAM initialization in hw/ppc4xx_devs.c

I was trying to launch a PowerPC "bamboo" machine with more than 256MB of RAM
with qemu-system-ppc -M bamboo -kernel $kernel -initrd $ramdisk -m 512, but QEMU
would just hang. However, when I used -m 256, the machine would boot.

I looked through the code in hw/ and it seems there is an error when the
RAM memory is setup (if my understanding is correct).

After patching it, the machine launched and booted successfully with 512MB of
RAM.

Signed-off-by: Alin Tomescu <tomescu.alin@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>