OMAP2420: mailbox: fix IVA vs DSP IRQ numbering

The IRQ numbering for the IVA and DSP mailboxes was switched due
to the wrong ordering in the OMAP2 mbox list.  Switch the ordering
so DSP is first and matches all the other SoCs.

Tested on OMAP2420/n810.

Signed-off-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>