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: "BenoƮt 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>
diff --git a/arch/arm/boot/dts/am4372.dtsi b/arch/arm/boot/dts/am4372.dtsi
index 46660ff..4367f75 100644
--- a/arch/arm/boot/dts/am4372.dtsi
+++ b/arch/arm/boot/dts/am4372.dtsi
@@ -168,6 +168,7 @@
 			reg = <0x480C8000 0x200>;
 			interrupts = <GIC_SPI 77 IRQ_TYPE_LEVEL_HIGH>;
 			ti,hwmods = "mailbox";
+			#mbox-cells = <1>;
 			ti,mbox-num-users = <4>;
 			ti,mbox-num-fifos = <8>;
 			mbox_wkupm3: wkup_m3 {