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