bhyve: add cmdline option for TPM emulation

At the moment, only a TPM passthru is supported. The cmdline looks like:

-l tpm,passthru,/dev/tpm0

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D32961