Update _3gpp-common-subscription-control.yang

          created and deleted by the system or be pre-installed. 

          Whether and when to emit heartbeat notifications is controlled by

          HeartbeatControl. Subscription for heartbeat is not supported via the

          HeartbeatControl. A single HeartbeatControl instance may emit

          heartbeats for multiple MnS producers.  The source of the heartbeat

          is identified in the heartbeat notification.

          Subscription for heartbeat is not supported via the

          NtfSubscriptionControl.";

        max-elements 1;