dt-bindings: iio: adc: qcom,spmi-vadc: Allow 1/16 for pre-scaling

The channel ADC5_USB_IN_V_16 is using 1/16 pre-scaling on at least
pm7250b and pmi632. Allow that in the schema.

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230414-pmi632-v1-5-fe94dc414832@z3ntu.xyz


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