bhyve: allow adding FwCfg items to ACPI tables

A TPM has an event log. Therefore, qemu adds a FwCfg item and adds it to
an ACPI table. We like to use the same OVMF driver as qemu, so we should
do the same. This commit adds the ability to basl to do it.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D40451

(cherry picked from commit 4e46ab0ebe96b56d4fa7ea7df4a81cd533109f07)