wifi: mt76: mt792x: introduce mt792x-lib module

mt792x-lib module will contain the shared code between mt7921 and new
MT79 WiFi7 chipset

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>