mailbox: Make mbox_chan_ops const

The mailbox controller's channel ops ought to be read-only.  Update
all the mailbox drivers to make their mbox_chan_ops const as well.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Cc: Ashwin Chaugule <ashwin.chaugule@linaro.org>
Cc: Ley Foon Tan <lftan@altera.com>
Acked-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>