Use tcb0 for timers as selected in at91_dt_defconfig.

[tested on at91sam9x25ek and at91sam9x35ek]
Tested-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>