dt-bindings: iio: adc: Split out QCOM VADC channel properties

dt-bindings: iio: adc: Split out QCOM VADC channel properties

Split out the common channel properties for QCOM VADC devices into a
separate file so that it can be included as a reference for devices
using them. This will be needed for the upcoming ADC5 Gen3 binding
support patch, as ADC5 Gen3 also uses all of these common properties.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Jishnu Prakash <jishnu.prakash@oss.qualcomm.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>