Add driver for Synopsys Designware Watchdog timer.
This driver supports some arm and arm64 boards equipped with
"snps,dw-wdt"-compatible watchdog device.
Tested on RK3399-based board (RockPro64).
Once started watchdog device cannot be stopped.
Interrupt handler has mode to kick watchdog even when software does not do it
This can be controlled via sysctl: dev.dwwdt.prevent_restart.
Also - driver handles system shutdown and prevents from restart when system
is asked to reboot.
Submitted by: firstname.lastname@example.org
Differential Revision: https://reviews.freebsd.org/D26761