Add DummyFencePlugin

src/main/scala/vexriscv/plugin/DummyFencePlugin.scala

@@ -0,0 +1,22 @@
+package vexriscv.plugin
+
+import spinal.core._
+import vexriscv.{VexRiscv, _}
+
+class DummyFencePlugin extends Plugin[VexRiscv]{
+
+  override def setup(pipeline: VexRiscv): Unit = {
+    import Riscv._
+    import pipeline.config._
+
+    val decoderService = pipeline.service(classOf[DecoderService])
+    decoderService.add(FENCE_I, Nil)
+    decoderService.add(FENCE, Nil)
+  }
+
+  override def build(pipeline: VexRiscv): Unit = {
+    import pipeline._
+    import pipeline.config._
+    //Dummy
+  }
+}