dt-bindings: touchscreen: ti,am3359-tsc: New yaml description

This touchscreen controller is already described in a text file:
Documentation/devicetree/bindings/input/touchscreen/ti-tsc-adc.txt

After introducing a proper description of the MFD, this is the second
step. The file cannot be removed yet as it also contains an ADC
description.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211015081506.933180-4-miquel.raynal@bootlin.com