Add comment explaining CanHavePeripheryFFT

Co-authored-by: Abraham Gonzalez <abe.j.gonza@gmail.com>

generators/chipyard/src/main/scala/DigitalTop.scala

@@ -32,7 +32,7 @@ class DigitalTop(implicit p: Parameters) extends ChipyardSystem
   with chipyard.example.CanHavePeripheryStreamingPassthrough // Enables optionally adding the DSPTools streaming-passthrough example widget
   with nvidia.blocks.dla.CanHavePeripheryNVDLA // Enables optionally having an NVDLA
   with chipyard.clocking.HasChipyardPRCI // Use Chipyard reset/clock distribution
-  with fftgenerator.CanHavePeripheryFFT
+  with fftgenerator.CanHavePeripheryFFT // Enables optionally having an MMIO-based FFT block
 {
   override lazy val module = new DigitalTopModule(this)
 }