bhyve: make basl_table_add_* functions public

The code will be more readable if we use struct definitions from ACPI-CA
to build ACPI tables. We can fill out the struct and append it to the
basl_table by using basl_table_append_bytes. After that, we have to
declare which checksums, length and pointers should be patched by basl.
That's done by the add_* functions.

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/D37405

(cherry picked from commit 7263419f387db306340ed39fc058580872b64f3b)