mbox_test: updated to omap mailbox framework as of 3.8.13.

Adapted from a test program written by Kevin Hilman:
https://github.com/khilman/omap-test-mailbox

Added a timeout when send fails (as a result of the kfifo being full)
to allow the rx tasklet to run which will call the notifier callback, which
will empty the receive queue.

Tested on DRA7XX.

Signed-off-by: G Anthony <a0783926@ti.com>