ppc/pegasos2: Change default CPU to 7457

ppc/pegasos2: Change default CPU to 7457

Previously 7400 was selected as a safe choice as that is used by other
machines so it's better tested but AmigaOS does not know this CPU and
disables some features when running on it. The real hardware has
7447/7457 G4 CPU so change the default to match that now that it was
confirmed to work better with AmigaOS.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Tested-by: Rene Engel <ReneEngel80@emailn.de>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20230528152937.B8DAD74633D@zero.eik.bme.hu>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>