linux-firmware: split rtl8761 firmware

Realtek Bluetooth devices require binary firmware files. Package them
separately in order to avoid installing the full linux-firmware package
on embedded devices.

Affected (end user) products (incomplete list):
- TP-Link UB500
- Logilink BT0054

Signed-off-by: Christian Eggers <ceggers@arri.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>