Fix more bugs with arty100t

generators/chipyard/src/main/scala/Subsystem.scala

@@ -62,6 +62,9 @@ class ChipyardSubsystem(implicit p: Parameters) extends BaseSubsystem
     val intSink = IntSinkNode(IntSinkPortSimple())
     intSink := intNexus :=* ibus.toPLIC
 
+    // avoids a bug when there are no interrupt sources
+    ibus.fromAsync := NullIntSource()
+
     // Need to have at least 1 driver to the tile notification sinks
     tileHaltXbarNode := IntSourceNode(IntSourcePortSimple())
     tileWFIXbarNode := IntSourceNode(IntSourcePortSimple())