mt76: mt7615: add support for registering a second wiphy via debugfs

This is only used for testing for now. In the future it will be possible to
enable the second PHY through an proper API via device tree

Running:
 echo 1 > /sys/kernel/debug/ieee80211/phyX/mt76/dbdc

will register a second wiphy (and limit the primary one to 2 spatial streams).
The second wiphy will only be able to run on 5 GHz, while the primary one can
operate in both bands (if supported by the device).

Signed-off-by: Felix Fietkau <nbd@nbd.name>