ARM: dts: sun6i: Add resistive touchscreen controller node to dtsi

Now that we support the sun6i variant of the touchscreen controller,
add the device node to the dtsi so we can use it.

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