Enhanced DeviceDescription and added default implementation for it.
diff --git a/net/api/src/main/java/org/onlab/onos/net/device/DefaultPortDescription.java b/net/api/src/main/java/org/onlab/onos/net/device/DefaultPortDescription.java
new file mode 100644
index 0000000..c171098
--- /dev/null
+++ b/net/api/src/main/java/org/onlab/onos/net/device/DefaultPortDescription.java
@@ -0,0 +1,7 @@
+package org.onlab.onos.net.device;
+
+/**
+ * Default implementation of immutable port description.
+ */
+public class DefaultPortDescription implements PortDescription {
+}