bamboo, sam460ex: Tidy up error message for unsupported RAM size

bamboo, sam460ex: Tidy up error message for unsupported RAM size

Improve

    $ ppc-softmmu/qemu-system-ppc -M sam460ex -m 4096
    qemu-system-ppc: Max 1 banks of 2048 ,1024 ,512 ,256 ,128 ,64 ,32 MB DIMM/bank supported
    qemu-system-ppc: Possible valid RAM size: 2048

to

    qemu-system-ppc: at most 1 bank of 2048, 1024, 512, 256, 128, 64, 32 MiB each supported
    Possible valid RAM size: 1024 MiB

Signed-off-by: Markus Armbruster <[email protected]>
Message-Id: <20200422134815[email protected]>
Reviewed-by: BALATON Zoltan <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>