clocksource/drivers/timer-atmel-tcb: Rework 32khz clock selection

On all the supported SoCs, the slow clock is always ATMEL_TC_TIMER_CLOCK5,
avoid looking it up and pass it directly to setup_clkevents.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200710230813.1005150-6-alexandre.belloni@bootlin.com