Use tcb0 for timers as selected in at91_dt_defconfig.

Cc: Martin Reimann <martin.reimann@egnite.de>
Cc: Tim Schendekehl <tim.schendekehl@egnite.de>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>