watchdog: wdt-uclass.c: add wdt_stop_all() helper

watchdog: wdt-uclass.c: add wdt_stop_all() helper

Since the watchdog_dev member of struct global_data is going away in
favor of the wdt-uclass handling all watchdog devices, prepare for
that by adding a helper to call wdt_stop() on all known devices.

If an error is encountered, still do wdt_stop() on remaining devices,
but remember and return the first error seen.

Initially, this will only be used in one single
place (board/alliedtelesis/x530/x530.c).

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>