rtlbtfw: Firmware loader for Realtek 87XX/88XX bluetooth USB adaptors

Firmware files are available in the comms/rtlbt-firmware port.

Sponsored by: Future Crew LLC
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D46739

(cherry picked from commit 5036d9652a5701d00e9e40ea942c278e9f77d33d)