ARM: dts: add dm816x missing #mbox-cells

Add missing #mbox-cells for dm816x mbox DT node.

Cc: Brian Hutchinson <b.hutchman@gmail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

arch/arm/boot/dts/dm816x.dtsi

@@ -218,6 +218,7 @@ mailbox: mailbox@480c8000 {
 			reg = <0x480c8000 0x2000>;
 			interrupts = <77>;
 			ti,hwmods = "mailbox";
+			#mbox-cells = <1>;
 			ti,mbox-num-users = <4>;
 			ti,mbox-num-fifos = <12>;
 			mbox_dsp: mbox_dsp {