target/ppc/cpu-models: Re-group the 970 CPUs together again

target/ppc/cpu-models: Re-group the 970 CPUs together again

The addition of the POWER9 CPUs divided the entries for the 970 CPUs,
which is a little bit confusing when you look at the code. So let's
re-group the 970 CPUs together again, and since these chips have been
based on the POWER4 processor, move them also in front of the POWER5
chips now.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>