Documentation: dt-bindings: Document STM32 ADC DT bindings

This patch adds documentation of device tree bindings for the STM32 ADC.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>