iio: adc: add ADC driver for the TI TSC2046 controller

iio: adc: add ADC driver for the TI TSC2046 controller

Basically the TI TSC2046 touchscreen controller is 8 channel ADC optimized for
the touchscreen use case. By implementing it as an IIO ADC device, we can
make use of resistive-adc-touch and iio-hwmon drivers.

Polled readings are currently not implemented to keep this patch small, so
iio-hwmon will not work out of the box for now.

So far, this driver was tested with a custom version of resistive-adc-touch driver,
since it needs to be extended to make use of Z1 and Z2 channels. The X/Y
are working without additional changes.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20210428073208.19570-4-o.rempel@pengutronix.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>