ARM: dts: at91: sama5d4 Xplained: use TCB2 as timers

As TCB2 doesn't have any output pins, use for timers.

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>