bhyve: add basl support for generic addresses

In upcoming commits, bhyve will build some ACPI tables by it's own.
Therefore, it should be capable of appending GENERIC_ADDRESS structs to
ACPI tables.

Reviewed by: jhb, markj
Approved by: manu (mentor)
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D36988