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iwmbtfw: Firmware loader for Intel Wireless 8260 based Bluetooth USB devices

Currently supported models are: 8260, 8265, 9560, 9260 and 22161.
Firmware files can be installed with comms/iwmbt-firmware port.

PR: 237083
Reviewed by: hps, emax
X-MFC with: r351196
Differential Revision: https://reviews.freebsd.org/D21071