clocksource/drivers/timer-atmel-tcb: Stop using the 32kHz for clockevents

Stop using the slow clock as the clock source for 32 bit counters because
even at 10MHz, they are able to handle delays up to two minutes. This
provides a way better resolution.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200710230813.1005150-8-alexandre.belloni@bootlin.com