thermal: sun8i: Add hwmon support

Expose sun8i thermal as a HWMON device.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20191228171904.24618-1-tiny.windzz@gmail.com