thermal: rockchip: enhance the tsadc's bandgap feature for rk3399

Due to the voltage ripple, the sensing data of the tsadc is not accurate.
And in this patch, the bandgap feature is enhanced to remove the voltage
ripple, and then the tsadc can sense the temperature more precisely.

Obsolete codes are removed as well.

Signed-off-by: Rocky Hao <rocky.hao@rock-chips.com>
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Heiko Stuebner <heiko@sntech.de>
Cc: linux-pm@vger.kernel.org
Tested-by: Stephen Barber <smbarber@chromium.org>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>