dt-bindings: iio: adc: Add rtq6056 adc support

Add the documentation for Richtek rtq6056.

Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/1658242365-27797-2-git-send-email-u0084500@gmail.com


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