Update ssPBCH_BlockPower value in the conf file to match actual outpur power

- currently, -25 dBm/RE is the maximum measured power from USRP
  corresponding to att_tx = 0 dB, therefore ssPBCH_BlockPower has been
  updated accordingly
- this param is used at UE side for the pathloss computation, which now
  seems to match measurements observed during lab testing

targets/PROJECTS/GENERIC-LTE-EPC/CONF/gnb.band78.tm1.106PRB.usrpx300.conf

@@ -187,7 +187,7 @@ gNBs =
       nrofUplinkSlots                                               = 2;
       nrofUplinkSymbols                                             = 4;
 
-  ssPBCH_BlockPower                                             = 10;
+  ssPBCH_BlockPower                                                 = -25;  // -25 dBm/RE is the maximum measured power from USRP corresponding to att_tx = 0 dB
   }
 
   );