mailbox: create opened message type

mailbox: create opened message type

Current message type is a u32 to fit HW fifo format.
This should be extended to support any message exchanges
and type of mailbox.

Proposed structure owns the original u32 and an optional
pointer on additional data.

Adaptations made to remoteproc and tidspbridge drivers.

Signed-off-by: Loic Pallardy <loic.pallardy@st.com>
Signed-off-by: Omar Ramirez Luna <omar.ramirez@copitl.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Suman Anna <s-anna@ti.com>