mt76x0: pci: add fw uploading routine

Add pci load firmware routine. Move some shared mcu definitions
in mt76x02_mcu.h

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>