thermal: rockchip: add the notes for better reading

To update the notes for keeping in mind that quickly in case
someone re-read this driver in the future.

Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Cc: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>