arm64: dts: mediatek: mt8183: fix gce incorrect mbox-cells value

The binding documentation says:
- #mbox-cells: Should be 2.
	<&phandle channel priority>
	phandle: Label name of a gce node.
	channel: Channel of mailbox. Be equal to the thread id of GCE.
	priority: Priority of GCE thread.

Fix the value of #mbox-cells.

Fixes: d3c306e3 ("arm64: dts: add gce node for mt8183")
Signed-off-by: Fabien Parent <fparent@baylibre.com>
Link: https://lore.kernel.org/r/20201018194225.3361182-1-fparent@baylibre.comSigned-off-by: Matthias Brugger <matthias.bgg@gmail.com>

arch/arm64/boot/dts/mediatek/mt8183.dtsi

@@ -363,7 +363,7 @@ gce: mailbox@10238000 {
 			compatible = "mediatek,mt8183-gce";
 			reg = <0 0x10238000 0 0x4000>;
 			interrupts = <GIC_SPI 162 IRQ_TYPE_LEVEL_LOW>;
-			#mbox-cells = <3>;
+			#mbox-cells = <2>;
 			clocks = <&infracfg CLK_INFRA_GCE>;
 			clock-names = "gce";
 		};