Add sky130_fd_io__top_xres4v2 stub

verilog/stubs/sky130_fd_io__top_xres4v2.v

diff --git a/verilog/stubs/sky130_fd_io__top_xres4v2.v b/verilog/stubs/sky130_fd_io__top_xres4v2.v
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+++ b/verilog/stubs/sky130_fd_io__top_xres4v2.v
@@ -0,0 +1,52 @@
+/*
+ * Copyright 2020 The SkyWater PDK Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+*/
+
+// This stub is currently needed because the verilog files of the I/O are not
+// parsable yet by Yosys due to some simulation-related constructs
+(* blackbox *)
+module sky130_fd_io__top_xres4v2 ( TIE_WEAK_HI_H,  XRES_H_N, TIE_HI_ESD, TIE_LO_ESD,
+                                   AMUXBUS_A, AMUXBUS_B, PAD, PAD_A_ESD_H, ENABLE_H, EN_VDDIO_SIG_H, INP_SEL_H, FILT_IN_H,
+                                   DISABLE_PULLUP_H, PULLUP_H, ENABLE_VDDIO
+                                   ,VCCD, VCCHIB, VDDA, VDDIO,VDDIO_Q, VSSA, VSSD, VSSIO, VSSIO_Q, VSWITCH
+                                 );
+output XRES_H_N;
+inout AMUXBUS_A;
+inout AMUXBUS_B;
+inout PAD;
+input DISABLE_PULLUP_H;
+input ENABLE_H;
+input EN_VDDIO_SIG_H;
+input INP_SEL_H;
+input FILT_IN_H;
+inout PULLUP_H;
+input ENABLE_VDDIO;
+input VCCD;
+input VCCHIB;
+input VDDA;
+input VDDIO;
+input VDDIO_Q;
+input VSSA;
+input VSSD;
+input VSSIO;
+input VSSIO_Q;
+input VSWITCH;
+inout PAD_A_ESD_H;
+output TIE_HI_ESD;
+output TIE_LO_ESD;
+inout TIE_WEAK_HI_H;
+endmodule