arm: apple: Use watchdog timer for system reset

Rely on the new watchdog timer driver and the sysreset uclass to
reset the system.  This gets rid of hard-coded addresses and
should work on systems based on the new M1 Pro and M1 Max SoCs
as well.

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>