8f4cf30903 ("watchdog: Allow multiple watchdogs") introduced the ability
to set a per-watchdog priority from within drivers, which is usually
populated with of_get_watchdog_priority.

For debugging, it can be useful to query and override this priority on
the fly.  Provide a device parameter to do so.
As watchdog_get_default only considers priorities > 0, it makes sense
to have a newly set priority of 0 disable the watchdog.

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