mt76: add stbc entries to mt76_rate_power

Add stbc tx power eeprom parsing support for mt76x2 driver.
When writing power entries, make a distinction between rates that are
read from the same EEPROM value, but have separate register entries.

No effect on runtime behavior, but preparation for unification with mt76x0
and for placing restrictions on individual rate power limits

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