ARM: dts: DRA7: Add interrupts property to mailbox nodes

Add the interrupts property to all the 13 mailbox nodes in
DRA7xx. The interrupts property information added is inline
with the expected values with the DRA7xx crossbar driver,
and is common to both DRA74x and DRA72x SoCs.

Do note that the mailbox 1 is only capable of generating out
3 interrupts, while all the remaining mailboxes have 4
interrupts each.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>