mailbox: sun6i-msgbox: Add a new mailbox driver

Allwinner sun6i, sun8i, sun9i, and sun50i SoCs contain a hardware
message box used for communication between the ARM CPUs and the ARISC
management coprocessor. This mailbox contains 8 unidirectional
4-message FIFOs.

Add a driver for it, so it can be used with the Linux mailbox framework.

Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>