dt-bindings: watchdog: sun4i: Add the watchdog clock

The Allwinner watchdog has a clock that has been described in some DT, but
not all of them.

The binding is also completely missing that description. Let's add that
property to be consistent.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20190821143835.7294-4-mripard@kernel.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>