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.com


Signed-off-by: Joel Stanley <joel@jms.id.au>