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