bhyve: maintain RSDT and XSDT by basl

In a subsquent commit the TPM emulation will build it's own TPM2 table.
This needs to be registered to the RSDT and XSDT. Instead of making the
rsdt and xsdt variables global, we can simply add a helper to basl.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D40559