Primary clock source configured for this processor. If a T1 or BRI module is providing the clock, it reads 'Vs' or 'Vs(pp,pp,...)', where s is slot number and pp are the port numbers. If the VoIP module is providing the clock, it reads 'voip'. It reads 'None' if no primary clock source is configured. If cmgClockSourceControl is remote(2), access is read-only. If cmgClockSourceControl is local(1), access is read-write.