Move $dffe to dffs.{v,ys}

tests/ice40/dffs.v

@@ -1,5 +1,37 @@
-module top
+module dff
     ( input d, clk, output reg q );
 	always @( posedge clk )
             q <= d;
 endmodule
+
+module dffe
+    ( input d, clk, en, output reg q );
+    initial begin
+      q = 0;
+    end
+	always @( posedge clk )
+		if ( en )
+			q <= d;
+endmodule
+
+module top (
+input clk,
+input en,
+input a,
+output b,b1,
+);
+
+dff u_dff (
+        .clk (clk ),
+        .d (a ),
+        .q (b )
+    );
+
+dffe u_ndffe (
+        .clk (clk ),
+        .en (en),
+        .d (a ),
+        .q (b1 )
+    );
+
+endmodule