powerpc/perf/hv-24x7: Aggregate result elements on POWER9 SMT8

On POWER9 SMT8 the 24x7 API returns two result elements for physical core
and virtual CPU events and we need to add their counts to get the final
result.

Reviewed-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>