mt76: extend DT rate power limits to support 11ax devices

Enable parsing per-rate txpower limits from DT for 11ax chipsets.

Co-developed-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Tested-by: Evelyn Tsai <evelyn.tsai@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>