dt-bindings:iio:adc: add documentation for TI TSC2046 controller

Add a binding documentation for the TI TSC2046 touchscreen controllers
ADC functionality.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210428073208.19570-3-o.rempel@pengutronix.de


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>