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>

Documentation/devicetree/bindings/iio/adc/st,stm32-adc.txt

@@ -47,6 +47,12 @@ Required properties:
 Optional properties:
 - A pinctrl state named "default" for each ADC channel may be defined to set
   inX ADC pins in mode of operation for analog input on external pin.
+- booster-supply: Phandle to the embedded booster regulator that can be used
+  to supply ADC analog input switches on stm32h7 and stm32mp1.
+- vdd-supply: Phandle to the vdd input voltage. It can be used to supply ADC
+  analog input switches on stm32mp1.
+- st,syscfg: Phandle to system configuration controller. It can be used to
+  control the analog circuitry on stm32mp1.
 
 Contents of a stm32 adc child node:
 -----------------------------------