saw-central/Yosys: Support "$_BUF_" cell type in yosys import.

Fixes #2828.

Author: brianhuffman

saw-central/src/SAWCentral/Yosys/Cell.hs

@@ -300,6 +300,9 @@ primCellToMap sc c args =
     -- "$concat" -> _
     CellTypeDff -> pure Nothing
     CellTypeFf -> pure Nothing
+    CellTypeBUF ->
+      do res <- input "A"
+         output res
     CellTypeUnsupportedPrimitive _ -> pure Nothing
     CellTypeUserType _ -> pure Nothing
   where

saw-central/src/SAWCentral/Yosys/IR.hs

@@ -135,6 +135,7 @@ textToPrimitiveCellType = Map.fromList
   , ("$bmux"        , CellTypeBmux)
   , ("$dff"         , CellTypeDff)
   , ("$ff"          , CellTypeFf)
+  , ("$_BUF_"       , CellTypeBUF)
   ]
 
 -- | Mapping from primitive cell types to textual representation
@@ -183,6 +184,7 @@ data CellType
   | CellTypeBmux
   | CellTypeDff
   | CellTypeFf
+  | CellTypeBUF
   | CellTypeUnsupportedPrimitive Text
   | CellTypeUserType Text
   deriving (Eq, Ord)