openflow_input: add of_bsn_tlv_reference

This TLV allows referencing an entry in another gentable by key.
diff --git a/openflow_input/bsn_tlv b/openflow_input/bsn_tlv
index 16637cd..e25e2de 100644
--- a/openflow_input/bsn_tlv
+++ b/openflow_input/bsn_tlv
@@ -394,3 +394,11 @@
     uint16_t length;
     uint32_t value; /* Milliseconds */
 };
+
+/* A reference to an entry in another gentable */
+struct of_bsn_tlv_reference : of_bsn_tlv {
+    uint16_t type == 59;
+    uint16_t length;
+    uint16_t table_id;
+    list(of_bsn_tlv_t) key;
+};