mailbox: mailbox-test: add support for fasync/poll

Currently the read operation on the message debug file returns error if
there's no data ready to be read. It expects the userspace to retry if
it fails. Since the mailbox response could be asynchronous, it would be
good to add support to block the read until the data is available.

We can also implement poll file operations so that the userspace can
wait to become ready to perform any I/O.

This patch implements the poll and fasync file operation callback for
the test mailbox device.

Cc: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>