time/timecounter: Mark 1st argument of timecounter_cyc2time() as const

The timecounter is not modified in this function. Mark it as const.
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210303103544.994855-1-mkl@pengutronix.de

include/linux/timecounter.h

@@ -124,7 +124,7 @@ extern u64 timecounter_read(struct timecounter *tc);
  * This allows conversion of cycle counter values which were generated
  * in the past.
  */
-extern u64 timecounter_cyc2time(struct timecounter *tc,
+extern u64 timecounter_cyc2time(const struct timecounter *tc,
 				u64 cycle_tstamp);
 
 #endif

kernel/time/timecounter.c

@@ -76,7 +76,7 @@ static u64 cc_cyc2ns_backwards(const struct cyclecounter *cc,
 	return ns;
 }
 
-u64 timecounter_cyc2time(struct timecounter *tc,
+u64 timecounter_cyc2time(const struct timecounter *tc,
 			 u64 cycle_tstamp)
 {
 	u64 delta = (cycle_tstamp - tc->cycle_last) & tc->cc->mask;