Introduce Pi Register model to PI Framework

Change-Id: I7fae87d5b5ed5fff34b3addfc148cee6fc98137c
diff --git a/core/api/src/main/java/org/onosproject/net/pi/runtime/PiEntityType.java b/core/api/src/main/java/org/onosproject/net/pi/runtime/PiEntityType.java
index 5ef452b..a6c612b 100644
--- a/core/api/src/main/java/org/onosproject/net/pi/runtime/PiEntityType.java
+++ b/core/api/src/main/java/org/onosproject/net/pi/runtime/PiEntityType.java
@@ -41,5 +41,10 @@
     /**
      * Meter config.
      */
-    METER_CELL_CONFIG
+    METER_CELL_CONFIG,
+
+    /**
+     * Register entry.
+     */
+    REGISTER_CELL
 }