arm/omap: use system_wq in mailbox

With cmwq, there's no reason to use a separate workqueue for mailbox.
Use the system_wq instead.  mbox->rxq->work is sync flushed in
omap_mbox_fini() to make sure it's not running on any cpu, which makes
sure that no mbox work is running when omap_mbox_exit() is entered.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Hari Kanigeri <hari.kanigeri@gmail.com>
Cc: Tony Lindgren <tony@atomide.com>
Cc: linux-omap@vger.kernel.org