mailbox: aspeed: add mailbox driver for AST27XX series SoC

Add mailbox controller driver for AST27XX SoCs, which provides
independent tx/rx mailbox between different processors. There are 4
channels for each tx/rx mailbox and each channel has an 32-byte FIFO.

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