Use tcb0 for timers as selected in at91_dt_defconfig.

Cc: Sergio Tanzilli <tanzilli@acmesystems.it>
Cc: Douglas Gilbert <dgilbert@interlog.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>