dt-bindings: iio: adc: ti,am3359-adc: New yaml description

This ADC was already described in a text file also containing an MFD
description an a touchscreen description:
Documentation/devicetree/bindings/input/touchscreen/ti-tsc-adc.txt
Let's add a proper description for this hardware.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211015081506.933180-5-miquel.raynal@bootlin.com