modules: tf-m: fix Ethos-U Cmake warnings

This is to fix the below compiler warning from TF-M builds
   that don't use the Ethos-U driver.
```
CMake Warning:
  Manually-specified variables were not used by the project:

    ETHOS_DRIVER_PATH
```

Signed-off-by: Sudan Landge <sudan.landge@arm.com>