arch: x86: lib: acpi_table: Fix MCFG entries

Commit d9531375 ("x86: Move SSDT table to a writer function")
introduced a bug where the actual MCFG entries are no longer generated.

Cc: Simon Glass <sjg@chromium.org>
Fixes: d9531375 ("x86: Move SSDT table to a writer function")
Signed-off-by: Moritz Fischer <moritzf@google.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>