dt-bindings: arm: stm32: Document Avenger96 devicetree binding

This commit documents Avenger96 devicetree binding based on
STM32MP157 SoC.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>