clocksource/drivers/timer-atmel-tcb: Fill tcb_config

Use the tcb_config and struct atmel_tcb_config to get the timer counter
width. This is necessary because atmel_tcb_config will be extended later
on.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200710230813.1005150-7-alexandre.belloni@bootlin.com