Following 4806e1087e ,
the following warning appears for some boards (e.g. `qemu_cortex_a53`)
```
CMake Warning at /zephyr/CMakeLists.txt:798 (message):
No SOURCES given to Zephyr library: drivers__cache
Excluding target from build.
```
Allow this driver to have no sources.
Signed-off-by: Henri Xavier <datacomos@huawei.com>