iio: adc: stm32-adc: add analog switches supply control

On stm32h7 and stm32mp1, the ADC inputs are multiplexed with analog
switches which have reduced performances when their supply is below 2.7V
(vdda by default):
- 3.3V embedded booster can be used, to get full ADC performances
  (increases power consumption).
- vdd supply can be selected if above 2.7V by setting ANASWVDD syscfg bit,
  on STM32MP1 only.

Make this optional, since this is a trade-off between analog performance
and power consumption.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>