mailbox: omap: Fix mailbox interrupt sharing

Multiple mailbox users can share one interrupt line. This flag was
mistakenly dropped as part of the FIFO removal. Mark the IRQ as shared.

Reported-by: Beleswar Padhi <b-padhi@ti.com>
Fixes: 3f58c1f4 ("mailbox: omap: Remove kernel FIFO message queuing")
Signed-off-by: Andrew Davis <afd@ti.com>
Tested-by: Beleswar Padhi <b-padhi@ti.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>