dt-bindings: arm: aspeed: document Ampere Mt.Mitchell BMC compatibles

Document Ampere Mt.Mitchell BMC board compatible.
Signed-off-by: Quan Nguyen <quan@os.amperecomputing.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220817071539.176110-2-quan@os.amperecomputing.comSigned-off-by: Joel Stanley <joel@jms.id.au>

Documentation/devicetree/bindings/arm/aspeed/aspeed.yaml

@@ -69,6 +69,7 @@ properties:
       - description: AST2600 based boards
         items:
           - enum:
+              - ampere,mtmitchell-bmc
               - aspeed,ast2600-evb
               - aspeed,ast2600-evb-a1
               - facebook,bletchley-bmc