timecounter: Merge FreeBSD change r304285

Constify the pointers to eventtimer and timecounter name strings.

The need for this appears as soon as you try to set the names to something
that isn't a "quoted literal".  (I'm actually confused why quoted strings
aren't a problem as well, we must have some warning disabled.)

Update #3175.

cpukit/score/include/sys/timetc.h

@@ -51,7 +51,7 @@ struct timecounter {
 		/* This mask should mask off any unimplemented bits. */
 	uint64_t		tc_frequency;
 		/* Frequency of the counter in Hz. */
-	char			*tc_name;
+	const char		*tc_name;
 		/* Name of the timecounter. */
 	int			tc_quality;
 		/*