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>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>