mailbox: add CIX mailbox driver

mailbox: add CIX mailbox driver

The CIX mailbox controller, used in the Cix SoCs, like sky1.
facilitates message transmission between multiple processors
within the SoC, such as the AP, PM, audio DSP, SensorHub MCU,
and others.

Acked-by: Jassi Brar <jassisinghbrar@gmail.com>
Reviewed-by: Peter Chen <peter.chen@cixtech.com>
Signed-off-by: Guomin Chen <Guomin.Chen@cixtech.com>
Signed-off-by: Gary Yang <gary.yang@cixtech.com>
Signed-off-by: Lihua Liu <Lihua.Liu@cixtech.com>
Signed-off-by: Peter Chen <peter.chen@cixtech.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>