dt-bindings: iio: adc: stm32: 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). Booster or vdd can be used, to get full ADC
performances.

Add documentation for:
- optional booster-supply that can be used on stm32h7 and stm32mp1.
- optional vdd-supply that can be used on stm32mp1 to supply analog
  circuitry, selected by syscfg.
Mark these as 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>