logging: Handle 0 Hz frequency in log_output

It is possible that user provides 0 hz frquency to the log_output
(if CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC=0). In that case timestamp
must not be formatted to avoid division by 0.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>