The ESP32 Ethernet Kit has only a single GPIO pin exposed to SAUL via
`saul_gpio`, but that GPIO doubles as PHY clock input when `esp32_eth`
is used. Hence, the `saul_gpio` dep should be optional and only kick
in when `esp32_eth` is not used.
19953: boards/esp32s3-wt32-sc01-plus: fix Kconfig r=aabadie a=gschorcht
### Contribution description
This PR fixes a remaining Kconfig mismatch. It should fix the last compilation problem of the nightly.
### Testing procedure
```
python3 dist/tools/compile_test/compile_like_murdock.py -a tests/drivers/ili9341/ -b esp32s3-wt32-sc01-plus
```
should fail w/o this PR but should succeed with this PR.
### Issues/PRs references
Co-authored-by: Gunar Schorcht <gunar@schorcht.net>
