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.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>