ARM: dts: OMAP2: Fix interrupts for OMAP2420 mailbox

The mailbox module is capable of generating two interrupts
to MPU in OMAP2420, compared to one in OMAP2430. The second
interrupt is to handle interrupts from the additional IVA
processor present only on OMAP2420.

Move the current common mailbox DT node into the SoC
specific files to allow the above differentiation. Also,
added back the interrupt-names on OMAP2420, that were
previously defined in hwmod data.

This fixes regression caused by the recent dropping of
hwmod data in favor for defining it in the .dts files.

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