Support watchdog depending on "mrvl,has-wdt" property

With this commit, watchdog is supported only in case of having
"mrvl,has-wdt" property or dedicated for watchdog compatibility field
("marvell,armada-380-wdt"). There is no need to modify dts files,
as "has-wdt" property already exists.

Obtained from: Semihalf
Sponsored by: Stormshield
Submitted by: Bartosz Szczepanek <bsz@semihalf.com>
Differential revision: https://reviews.freebsd.org/D4424