Input: sun4i-ts - add thermal zone sensor support

The touchscreen controller has a temperature sensor embedded in the SoC,
which already has hwmon support in the driver.

Add DT thermal zone support so we can use it with cpufreq for thermal
throttling.

This also adds a comment stating that we do not know the actual formula
for calculating the temperature.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>