dt-bindings: iio/adc: qcom,spmi-iadc: use double compatibles

As in other bindings, let's use specific compatibles together with the
fallback compatible. Adjust the bindings for it.

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221031182456.952648-1-luca@z3ntu.xyz


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