dt-bindings: soc: sophgo: add Milk-V Duo S board compatibles

Document the compatible strings for the Milk-V Duo S board [1]
which uses the SOPHGO SG2000 SoC.

Link: https://milkv.io/duo-s [1]

Signed-off-by: Joshua Milas <josh.milas@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Link: https://patch.msgid.link/20260530173347.33533-2-josh.milas@gmail.com
Signed-off-by: Inochi Amaoto <inochiama@gmail.com>
Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

Documentation/devicetree/bindings/soc/sophgo/sophgo.yaml

@@ -31,6 +31,10 @@ properties:
               - milkv,duo-module-01-evb
           - const: milkv,duo-module-01
           - const: sophgo,sg2000
+      - items:
+          - enum:
+              - milkv,duo-s
+          - const: sophgo,sg2000
       - items:
           - enum:
               - sipeed,licheerv-nano-b