mailbox/omap: check for any unread messages during suspend

The OMAP mailbox driver is used by clients to communicate with remote
processors in general. The mailbox clients are expected to have stopped
communicating with these remote processors during a system suspend. The
OMAP mailbox fifos are expected to not have any messages as such. Add a
check for any pending unprocessed messages in the suspend callback, to
detect any communication protocol issues of the mailbox clients. The
system suspend is aborted if any messages are found.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>