dt-bindings: iio: adc: Add TI TWL603X GPADC

Document TI TWL603X GPADC devicetree bindings.
A driver is already there, the compatibles are used, but not documented.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230820123229.530371-1-andreas@kemnade.info


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