dt-bindings: arm: amlogic: add board AN400

Add the board AN400 tree bindings based Amloigc T7 SoC.
Signed-off-by: Xianwei Zhao <xianwei.zhao@amlogic.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230706091954.3301224-2-xianwei.zhao@amlogic.comSigned-off-by: Neil Armstrong <neil.armstrong@linaro.org>

Documentation/devicetree/bindings/arm/amlogic.yaml

@@ -221,6 +221,7 @@ properties:
       - description: Boards with the Amlogic T7 A311D2 SoC
         items:
           - enum:
+              - amlogic,an400
               - khadas,vim4
           - const: amlogic,a311d2
           - const: amlogic,t7