target-ppc: Turn descriptive CPU family comments into device descriptions

This gets rid of some more overly long comments that have lost most of
their purpose now that in most cases there's only two functions left per
CPU family.

The class field is inherited by the actual CPU models, so override it.
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Signed-off-by: NAlexander Graf <agraf@suse.de>