Enhancing driver subsystem to support asynchronous event notifications.

Change-Id: I6850aae4f660b8328378da98460529eb58531732
diff --git a/core/api/src/main/java/org/onosproject/net/driver/DriverRegistry.java b/core/api/src/main/java/org/onosproject/net/driver/DriverRegistry.java
index 36a7642..99152c6 100644
--- a/core/api/src/main/java/org/onosproject/net/driver/DriverRegistry.java
+++ b/core/api/src/main/java/org/onosproject/net/driver/DriverRegistry.java
@@ -15,12 +15,15 @@
  */
 package org.onosproject.net.driver;
 
+import org.onosproject.event.ListenerService;
+
 import java.util.Set;
 
 /**
  * Service for obtaining drivers and driver behaviour implementations.
  */
-public interface DriverRegistry extends DriverPrimordialResolver, DriverResolver {
+public interface DriverRegistry extends DriverPrimordialResolver, DriverResolver,
+        ListenerService<DriverEvent, DriverListener> {
 
     /**
      * Returns the overall set of drivers being provided.