Adding NONE as subcategory as it is an optional field for rpc events

Change-Id: I615ba1f1e67ea64a5d3edf3912d64e64e3555203
diff --git a/protos/voltha_protos/events.proto b/protos/voltha_protos/events.proto
index 4dd8347..5092bab 100644
--- a/protos/voltha_protos/events.proto
+++ b/protos/voltha_protos/events.proto
@@ -177,6 +177,7 @@
         ONT = 2;
         ONU = 3;
         NNI = 4;
+        NONE = 5; //Adding None for RPC Events
         // Add new event categories here.
     }
 }