dt-bindings: document Rockchip thermal

This add the necessary binding documentation for the thermal
found on Rockchip SoCs

Signed-off-by: zhaoyifeng <zyf@rock-chips.com>
Signed-off-by: Caesar Wang <caesar.wang@rock-chips.com>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>