ARM: dts: OMAP2+: Add #mbox-cells property to all mailbox nodes

The '#mbox-cells' property is added to all the OMAP mailbox
nodes. This property is mandatory with the new mailbox framework.

Cc: "Benoît Cousson" <bcousson@baylibre.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>