Enhancing driver subsystem to support asynchronous event notifications.

Change-Id: I6850aae4f660b8328378da98460529eb58531732
diff --git a/core/net/src/main/java/org/onosproject/net/driver/impl/DriverManager.java b/core/net/src/main/java/org/onosproject/net/driver/impl/DriverManager.java
index c96b6ef..47d4f91 100644
--- a/core/net/src/main/java/org/onosproject/net/driver/impl/DriverManager.java
+++ b/core/net/src/main/java/org/onosproject/net/driver/impl/DriverManager.java
@@ -31,6 +31,7 @@
 import org.onosproject.net.driver.DefaultDriverHandler;
 import org.onosproject.net.driver.Driver;
 import org.onosproject.net.driver.DriverHandler;
+import org.onosproject.net.driver.DriverListener;
 import org.onosproject.net.driver.DriverRegistry;
 import org.onosproject.net.driver.DriverService;
 import org.slf4j.Logger;
@@ -129,4 +130,13 @@
         return new DefaultDriverHandler(new DefaultDriverData(driver, deviceId));
     }
 
+    @Override
+    public void addListener(DriverListener listener) {
+        registry.addListener(listener);
+    }
+
+    @Override
+    public void removeListener(DriverListener listener) {
+        registry.removeListener(listener);
+    }
 }