dt-bindings: Document STM32 ADC DT bindings

This patch adds documentation of device tree bindings for the STM32 ADC.
It's based on linux-v4.18-rc* dt-bindings, at the time of writing:
- Documentation/devicetree/bindings/iio/adc/st,stm32-adc.txt

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>