ARM: dts: sunxi: Enable thermal sensor support for RTP on sun[457]i

Now that the resistive touchpanel driver supports thermal sensors,
add the "#thermal-sensor-cells" property as required by the thermal
framework.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>