dt-bindings: iio/adc: qcom,spmi-iadc: document PM8226 compatible

Document the compatible for the IADC used on PM8226.

Signed-off-by: Rayyan Ansari <rayyan@ansari.sh>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221228230421.56250-5-rayyan@ansari.sh


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