Add support for watchdog on Armada38x

Add support for watchdog on Armada38x

A38X watchdog support was implemented in sys/arm/mv/timer.c driver.
It required following modifications:
- add "marvell,armada-380-wdt" compatibility, which supports only watchdog
- correct and enhance definitions related to timer control register
- unmask reset capability in RSTOUTn_MASK register
- use dedicated watchdog timer on A38X instead of second timer

Obtained from:  Semihalf
Sponsored by:   Stormshield
Submitted by:   Bartosz Szczepanek <bsz@semihalf.com>
Differential revision:  https://reviews.freebsd.org/D4423