v2x: Fix the clock output detection so that dsp_in_registered test can be enabled

https://github.com/SymbiFlow/symbiflow-arch-defs/blob/5fbb149e1638223e9ea57ed65476ab8777febcdd/utils/vlog/tests/CMakeLists.txt#L7-L8

```
[100%] Generating dsp_in_registered_model_diff.diff
cd /usr/local/google/home/tansell/github/SymbiFlow/symbiflow-arch-defs/build/utils/vlog/tests/dsp_in_registered && diff -u /usr/local/google/home/tansell/github/SymbiFlow/symbiflow-arch-defs/build/utils/vlog/tests/dsp_in_registered/dsp_in_registered.model.golden.xml /usr/local/google/home/tansell/github/SymbiFlow/symbiflow-arch-defs/build/utils/vlog/tests/dsp_in_registered/dsp_in_registered.model.actual.xml > dsp_in_registered_model_diff.diff || true
cd /usr/local/google/home/tansell/github/SymbiFlow/symbiflow-arch-defs/build/utils/vlog/tests/dsp_in_registered && diff -u /usr/local/google/home/tansell/github/SymbiFlow/symbiflow-arch-defs/build/utils/vlog/tests/dsp_in_registered/dsp_in_registered.model.golden.xml /usr/local/google/home/tansell/github/SymbiFlow/symbiflow-arch-defs/build/utils/vlog/tests/dsp_in_registered/dsp_in_registered.model.actual.xml
--- /usr/local/google/home/tansell/github/SymbiFlow/symbiflow-arch-defs/build/utils/vlog/tests/dsp_in_registered/dsp_in_registered.model.golden.xml 2019-05-09 07:59:49.655000023 -0700
+++ /usr/local/google/home/tansell/github/SymbiFlow/symbiflow-arch-defs/build/utils/vlog/tests/dsp_in_registered/dsp_in_registered.model.actual.xml 2019-05-09 07:59:49.547001309 -0700
@@ -27,7 +27,7 @@



-
+



utils/vlog/tests/dsp_in_registered/CMakeFiles/dsp_in_registered_model_diff.dir/build.make:63: recipe for target 'utils/vlog/tests/dsp_in_registered/dsp_in_registered_model_diff.diff' failed
make[2]: *** [utils/vlog/tests/dsp_in_registered/dsp_in_registered_model_diff.diff] Error 1
make[2]: *** Deleting file 'utils/vlog/tests/dsp_in_registered/dsp_in_registered_model_diff.diff'
make[2]: Leaving directory '/usr/local/google/home/tansell/github/SymbiFlow/symbiflow-arch-defs/build'
CMakeFiles/Makefile2:21236: recipe for target 'utils/vlog/tests/dsp_in_registered/CMakeFiles/dsp_in_registered_model_diff.dir/all' failed
make[1]: *** [utils/vlog/tests/dsp_in_registered/CMakeFiles/dsp_in_registered_model_diff.dir/all] Error 2
make[1]: Leaving directory '/usr/local/google/home/tansell/github/SymbiFlow/symbiflow-arch-defs/build'
Makefile:140: recipe for target 'all' failed
make: *** [all] Error 2
```

This is the same type of test as https://github.com/SymbiFlow/symbiflow-arch-defs/issues/630