dt-bindings: watchdog: Add Apple Watchdog

Apple SoCs come with a simple embedded watchdog. This watchdog is also
required in order to reset the SoC.

Reviewed-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sven Peter <sven@svenpeter.dev>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Hector Martin <marcan@marcan.st>