dt-bindings: iio: adc: document qcom-spmi-rradc

Add dt-binding docs for the Qualcomm SPMI RRADC found in PMICs like
PMI8998 and PMI8994

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220429220904.137297-5-caleb.connolly@linaro.org


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