arch/x86/acpi: Consolidate POST code handling

Move ASL POST code declarations into a common file to avoid redundancy.
Also, provide a dummy implementation when `POST_IO` is not enabled, as
the value of `CONFIG_POST_IO_PORT` can't be used.

Change-Id: I891bd8754f10f16d618e76e1ab88c26164776a50
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/63988
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Reviewed-by: Lean Sheng Tan <sheng.tan@9elements.com>

src/arch/x86/acpi/post.asl

@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#if CONFIG(POST_IO)
+
+/* POST code support, typically on port 80 */
+OperationRegion (POST, SystemIO, CONFIG_POST_IO_PORT, 1)
+Field (POST, ByteAcc, Lock, Preserve)
+{
+	DBG0, 8
+}
+
+#else
+
+/* Dummy placeholder to avoid issues */
+Name (DBG0, 0)
+
+#endif