uboot-imx

mailbox: Allow attaching private data for mbox_chan

Sometimes mbox controllers wants to store private data in
mbox_chan so that it can be used at a later point of time.
Adding support for hooking private data.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>