soc: nrf53: Warn if workaround for anomaly 160 cannot be applied

This is a follow-up to commit fe3b97a8.

Add a cmake warning issued when the workaround for the nRF5340 anomaly
160 cannot be applied because the application is configured with no
system clock.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>