adding external ref to config file

targets/PROJECTS/GENERIC-LTE-EPC/CONF/gnb.band78.tm1.106PRB.usrpn300.conf

@@ -254,7 +254,7 @@ RUs = (
          max_pdschReferenceSignalPower = -27;
          max_rxgain                    = 114;
          eNB_instances  = [0];
-	 sdr_addrs = "addr=192.168.10.2,second_addr=192.168.20.2,mgmt_addr=192.168.100.8";
+	 sdr_addrs = "addr=192.168.10.2,second_addr=192.168.20.2,mgmt_addr=192.168.100.8,clock_source=external,time_source=external";
 
     }
 );