ichwd: introduce i6300esbwd watch dog driver

The intel 6300ESB watchdog is a special ICH-based watchdog device with a
different interface.
QEMU implements this watchdog for x86 systems.

This change enables watchdog mode (rather than free-running mode) and
introduces 1 sysctl:

• hw.i6300esbwd.0.locked: locks the watchdog register after the event is

triggered, preventing it from being disabled until a hard reset.

This feature has been tested on a Vultr AMD guest machine and local qemu
machine.

PR: 259673
Approved by: markj (mentor), lwhsu (mentor)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52049