mediatek: Add MT6639 (MT7927) Bluetooth firmware

Add the Bluetooth firmware for the MediaTek MT6639 (MT7927 Filogic 380)
combo WiFi 7 + BT 5.4 module. The BT subsystem uses hardware variant
0x6639 and connects via USB.

The firmware is loaded by the btmtk driver as:
  mediatek/mt6639/BT_RAM_CODE_MT6639_2_1_hdr.bin

Extracted from the ASUS ROG Crosshair X870E Hero driver package
(DRV_WiFi_MTK_MT7925_MT7927_TP_W11_64_V5603998_20250709R.zip).

Known hardware:
- ASUS ROG Crosshair X870E Hero (USB 0489:e13a)
- ASUS ProArt X870E-Creator WiFi (USB 13d3:3588)
- ASUS X870E-E (USB 13d3:3588)
- Gigabyte Z790 AORUS MASTER X (USB 0489:e10f)
- Lenovo Legion Pro 7 16ARX9 (USB 0489:e0fa)
- TP-Link Archer TBE550E PCIe (USB 0489:e116)

Link: https://bugzilla.kernel.org/show_bug.cgi?id=221096
Link: https://github.com/openwrt/mt76/issues/927
Signed-off-by: Javier Tia <floss@jetm.me>

WHENCE

@@ -6347,6 +6347,14 @@ Licence: Redistributable. See LICENCE.mediatek for details.
 
 --------------------------------------------------------------------------
 
+Driver: mt7927 - MediaTek MT7927 (MT6639) bluetooth chipset
+
+File: mediatek/mt6639/BT_RAM_CODE_MT6639_2_1_hdr.bin
+
+Licence: Redistributable. See LICENCE.mediatek for details.
+
+--------------------------------------------------------------------------
+
 Driver: mtk-2p5ge - MediaTek MT798x Internal 2.5G Ethernet Phy
 
 File: mediatek/mt7988/i2p5ge-phy-pmb.bin