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.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>