arm: dts: apple: Add watchdog timer node

Add a node for the watchdog timer based on the proposed Linux
device tree bindings.

Remove the old reboot node which was a watchdog timert node in
disguise using a preliminary device tree binding.

Signed-off-by: Sven Peter <sven@svenpeter.dev>
Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-on: Apple M1 Macbook
Tested-by: Simon Glass <sjg@chromium.org>