mt76: mt7615: add thermal sensor device support

Similar to mt7915, switching to use standard hwmon sysfs.
For reading temperature, cat /sys/class/ieee80211/phy*/hwmon*/temp1_input

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