MmuPlugin: fix generation without writeBack stage

If there is no writeBack stage, the elaboration step would hit a
NullPointerException when trying to insert into the writeBack stage.

Instead, pull from the most recent stage, which is where MMU access
should reside.

Signed-off-by: Sean Cross <sean@xobs.io>

src/main/scala/vexriscv/plugin/MmuPlugin.scala

@@ -250,8 +250,9 @@ class MmuPlugin(ioRange : UInt => Bool,
       }
     }
 
-    writeBack plug new Area{
-      import writeBack._
+    val fenceStage = stages.last
+    fenceStage plug new Area{
+      import fenceStage._
       when(arbitration.isValid && input(IS_SFENCE_VMA)){ // || csrService.isWriting(CSR.SATP)
         for(port <- core.ports; line <- port.cache) line.valid := False //Assume that the instruction already fetched into the pipeline are ok
       }