dt-bindings: iio: adc: add Texas Instruments ADS7924

Add device tree bindings document for the Texas Instruments ADS7924
ADC.

Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230115170623.3680647-3-hugo@hugovil.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>