mt76: mt7915: add thermal sensor device support

This provides userspace with a unified interface, hwmon sysfs, to monitor
temperature in the hardware and can be adapted to system monitoring tools.

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>