dt-bindings: mailbox: Add ASPEED AST2700 series SoC

Introduce the mailbox module for AST27XX series SoC, which is responsible
for interchanging messages between asymmetric processors.

Signed-off-by: Jammy Huang <jammy_huang@aspeedtech.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Andrew Jeffery <andrew@codeconstruct.com.au>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>