The Spreadtrum mailbox controller supports 8 channels to communicate
with MCUs, and it contains 2 different parts: inbox and outbox, which
are used to send and receive messages by IRQ mode.
Signed-off-by: NBaolin Wang <baolin.wang@unisoc.com>
Signed-off-by: NJassi Brar <jaswinder.singh@linaro.org>