rocket-dsptools DmaSpec test failure with current rocket-chip

[ucbjrl]

With current rocket-chip, after #185 is applied, DmaSpec.elaborate connected to the pbus fails with:

[info] - should elaborate connected to the pbus *** FAILED ***
[info]   firrtl.options.OptionsException: Exception thrown when elaborating ChiselGeneratorAnnotation
[info]   at chisel3.stage.ChiselGeneratorAnnotation.elaborate(ChiselAnnotations.scala:55)
[info]   ...
[info]   Cause: java.lang.IllegalArgumentException: requirement failed: pbus_clock_groups.node (A nexus node with parent 'pbus_clock_groups/pbus/lazyDut' at  (BusWrapper.scala:31:48)) has 1 required outputs and no possible inputs
[info]   at scala.Predef$.require(Predef.scala:281)
[info]   at freechips.rocketchip.diplomacy.MixedNexusNode.resolveStar(Nodes.scala:588)