thermal/drivers/mediatek: Add sensors-support

Add HWMON-support to mediateks thermal driver to allow lm-sensors
userspace tools read soc temperature

Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210608154530.70074-1-linux@fw-web.de