Since commit 46a99c9f ("s390x/cpumodel: model PTFF subfunctions
for Multiple-epoch facility") -cpu help no longer shows the MSA8
feature group. Turns out that we forgot to add the new MEPOCH_PTFF
group enum.

Fixes: 46a99c9f ("s390x/cpumodel: model PTFF subfunctions for Multiple-epoch facility")
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>