dt-bindings: watchdog: atmel: Add missing 'interrupts' property

With 'unevaluatedProperties' support implemented, the atmel,sama5d4-wdt
example has the following warning:

/home/rob/proj/git/linux-dt/.build-arm64/Documentation/devicetree/bindings/watchdog/atmel,sama5d4-wdt.example.dt.yaml: watchdog@fc068640: Unevaluated properties are not allowed ('interrupts' was unexpected)

Document the missing 'interrupts' property.

Cc: Wim Van Sebroeck <wim@linux-watchdog.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Cc: Eugen Hristev <eugen.hristev@microchip.com>
Cc: linux-watchdog@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20211206174045.2294873-1-robh@kernel.org