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
properly.
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: kjopek@gmail.com
Differential Revision: https://reviews.freebsd.org/D26761