soc: nrf53: Change logging level of anomaly 160 message to DEBUG

This is a follow-up to commit fe3b97a8.

This message should not be a warning, as it does not actually indicate
that something potentially bad happened. On the contrary, it informs
that conditions in which the anomaly 160 could occur were detected and
the anomaly was prevented from occurring. There is no need for this
message to appear in the default configuration (INFO level). In fact,
the message would undesirably flood the console in some cases (like
the kernel/mem_protect/stack_random test) and sometimes it would also
require enlarging the stack of the idle thread (the function is called
underneath k_cpu_idle()). Therefore, the logging level of this message
is changed to DEBUG.

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