logging: add log_output_timestamp_to_us() helper function

So far we only have log_msg_timestamp_get() function, which returns
internal timestamp representation. This is either clock cycles or uptime
in ms, depending on main clock precision.

Introduce log_output_timestamp_to_us() helper function, which allows to
convert internal logging timestamp to us.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>

subsys/logging/log_output.c

@@ -753,3 +753,10 @@ void log_output_timestamp_freq_set(uint32_t frequency)
 
 	freq = frequency;
 }
+
+uint64_t log_output_timestamp_to_us(uint32_t timestamp)
+{
+	timestamp /= timestamp_div;
+
+	return ((uint64_t) timestamp * 1000000U) / freq;
+}