serving: Tie off extension interface

serving/serving.v

@@ -230,6 +230,13 @@ module serving
       .o_dbus_we   (wb_dbus_we),
       .o_dbus_cyc  (wb_dbus_stb),
       .i_dbus_rdt  (wb_dbus_rdt),
-      .i_dbus_ack  (wb_dbus_ack));
+      .i_dbus_ack  (wb_dbus_ack),
+
+      .o_ext_funct3 (),
+      .i_ext_ready  (1'b0),
+      .i_ext_rd     (32'd0),
+      .o_ext_rs1    (),
+      .o_ext_rs2    (),
+      .o_mdu_valid  ());
 
 endmodule