input: touchscreen: resistive-adc-touch: add generic resistive ADC touchscreen

This adds a generic resistive touchscreen (GRTS) driver, which is based
on an IIO device (an ADC). It must be connected to the channels of an ADC
to receive touch data. Then it will feed the data into the input subsystem
where it registers an input device.
It uses an IIO callback buffer to register to the IIO device

Some parts of this patch are based on initial original work by
Mohamed Jamsheeth Hajanajubudeen and Bandaru Venkateswara Swamy

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>