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>

Documentation/devicetree/bindings/arm/stm32/stm32.yaml

@@ -25,5 +25,7 @@ properties:
           - const: st,stm32h743
 
       - items:
+          - enum:
+              - arrow,stm32mp157a-avenger96 # Avenger96
           - const: st,stm32mp157
 ...