sbus => pbus: 2 buffers should already be enough

There is a buffer on the sbus backside.
There is a buffer on the pbus frontside.

Between them is only an AtomicAutomata.
That should be enough for most designs.

src/main/scala/coreplex/SystemBus.scala

@@ -37,7 +37,7 @@ class SystemBus(params: SystemBusParams)(implicit p: Parameters) extends TLBusWr
 
   def toSplitSlaves: TLOutwardNode = outwardSplitNode
 
-  def toPeripheryBus(addBuffers: Int): TLOutwardNode = {
+  def toPeripheryBus(addBuffers: Int = 0): TLOutwardNode = {
     val (in, out) = bufferChain(addBuffers, name = Some("pbus"))
     in := pbus_fixer.node
     out