mailbox: add shared memory mailbox type

mailbox: add shared memory mailbox type

Some mailboxes are made up of cross interrupts
and associated shared memory.
Shared memory mapping is fixed and cross interrupt/shared
memory relation make impossible the use of virtio.
Mailbox framework must be enough opened to support
any kind of mailbox.

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