ARM: dts: at91: at91rm9200ek: use TCB0 for timers

Use tcb0 for timers like selected in at91_dt_defconfig.

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>