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

The Allwinner watchdog has an interrupt, either shared or dedicated
depending on the SoC, 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/20190821143835.7294-3-mripard@kernel.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>