The barebox EFI documentation notes:

> Current linux kernel (v5.0) will execute ExitBootServices() during the
> early boot stage and thus will automatically disable the (U)EFI watchdog.
> Since it is the proper behavior according to the (U)EFI specification, it
> is impossible to protect full boot chain by using this watchdog only.
> It is recommended to use an alternative hardware watchdog

Heed the advice and bump down the EFI watchdog priority below the
watchdog priority default.
This ensures the EFI watchdog isn't inadvertently used if other
watchdogs are registered.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>