dt-bindings: input: touchscreen: resistive-adc-touch: create bindings

Added bindings for generic resistive touchscreen ADC.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>