dt-bindings: input: touschcreen: Remove sun4i documentation

This patch removes the sun4i touchscreen controller binding
documentation since it has been merged with the sun4i GPADC binding
documentation.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>