bhyve: allow device specific DSDT entries

This feature will be used by future commits to implement a device
specific method (_DSM) for TPM devices.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D39321

(cherry picked from commit e976464a50aceebf97fb3cc6b94058111fc40de1)