iio: adc: qcom-vadc: move several adc5 functions to common file

ADC-TM5 driver will make use of several functions from ADC5 driver. Move
them to qcom-vadc-common driver.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20201204025509.1075506-4-dmitry.baryshkov@linaro.org


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