mt76: mt7915: add thermal cooling device support

Thermal cooling device support is added to control the temperature by
throttling the data transmission for the given duration. Throttling is
done by adjusting Tx period by given percentage of time. The thermal
device allows user to configure duty cycle.

Throttling can be disabled by setting the duty cycle to 0. The cooling
device can be found under /sys/class/thermal/cooling_deviceX/.
Corresponding soft link to this device can be found under phy folder

To set duty cycle as 80%,
echo 80 > /sys/class/ieee80211/phy*/cooling_device/cur_state

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>