ARM: dts: at91: sama5d2: add slow clock to watchdog node

As the watchdog timer needs the slow clock, add it to the currently defined
wdt node.

Reported-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>