powerpc/powernv: Set PELTV for compound PEs

Commit 262af557 ("powerpc/powernv: Enable M64 aperatus for PHB3")
introduced compound PEs in order to support M64 aperatus on PHB3.
However, we never configured PELTV for compound PEs. The patch
fixes that by: parent PE can freeze all child compound PEs. Any
compound PE affects the group.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>