dt-bindings: riscv: Add Milk-V Duo board compatibles

Document the compatible strings for the Milk-V Duo board[1] which uses
the SOPHGO CV1800B SoC[2].

Link: https://milkv.io/duo [1]
Link: https://en.sophgo.com/product/introduce/cv180xB.html [2]
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Chen Wang <unicorn_wang@outlook.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>

Documentation/devicetree/bindings/riscv/sophgo.yaml

@@ -18,6 +18,10 @@ properties:
     const: '/'
   compatible:
     oneOf:
+      - items:
+          - enum:
+              - milkv,duo
+          - const: sophgo,cv1800b
       - items:
           - enum:
               - milkv,pioneer