mailbox/omap: store mailbox interrupt type in omap_mbox_device

The interrupt type used for identifying the layout of the interrupt
configuration registers between OMAP4+ SoCs and older SoCs is stored
only in the sub-mailbox structures for easier access. Store this type
in the the omap_mbox_device structure as well along with the other
global variables. This is being done to facilitate the context save
and restore of appropriate registers during system suspend/resume.

Signed-off-by: Suman Anna <s-anna@ti.com>