dt-bindings: riscv: microchip: document icicle reference design

Add a compatible for the icicle kit's reference design. This represents
the FPGA fabric's contents & is versioned to denote which release of the
reference design it applies to.
Acked-by: NKrzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: NConor Dooley <conor.dooley@microchip.com>
Acked-by: NRob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220509142610.128590-5-conor.dooley@microchip.comSigned-off-by: NPalmer Dabbelt <palmer@rivosinc.com>

Documentation/devicetree/bindings/riscv/microchip.yaml

@@ -20,6 +20,7 @@ properties:
     items:
       - enum:
           - microchip,mpfs-icicle-kit
+          - microchip,mpfs-icicle-reference-rtlv2203
       - const: microchip,mpfs
 
 additionalProperties: true