Support Milk-V Duo and Duo 256M boards

[vurdalakov]

• Board JSON file: https://github.com/kubuds/sophgo-arduino/releases/download/v0.2.7/package_sg200x_index.json
• GitHub repository: https://github.com/kubuds/sophgo-arduino
• Official instructions: https://milkv.io/docs/duo/getting-started/arduino
• Official product page: https://milkv.io/duo