acpi_sbl_wmi: add a driver to trigger SBL firmware updates

Expose a sysctl knob to inform the SBL bootloader that a firmware update is
available and should be applied on the next reboot.

See also: https://slimbootloader.github.io/security/firmware-update.html#triggering-firmware-update
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D47035