tilelink: split Acquire into Acquire{Block,Perm} (#1030)

We had planned for a while to add an 'Overwrite' message which obtains
permissions without requiring retrieval of data. This is useful whenever
a master knows it will completely replace the contents of a cache block.

Instead of calling it Overwrite, we decided to split the Acquire type.

If you AcquirePerm, you MUST Release and ProbeAck with Data.

src/main/scala/tilelink/RAMModel.scala

@@ -116,7 +116,7 @@ class TLRAMModel(log: String = "")(implicit p: Parameters) extends LazyModule
 
       when (a_fire) {
         // Record the request so we can handle it's response
-        assert (a.opcode =/= TLMessages.Acquire)
+        assert (a.opcode =/= TLMessages.AcquireBlock && a.opcode =/= TLMessages.AcquirePerm)
 
         // Mark the operation as valid
         valid(a.source) := Bool(true)