Documentation: watchdog: at91sam9_wdt: add clocks property

The watchdog has an input clock, the slow clock. It is required as it will
not function without it.

Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>