dt-bindings: mailbox: mtk-gce: fix incorrect mbox-cells value

As the binding documentation says, #mbox-cells must have a value of 2,
but the example use a value 3. The MT8173 device tree correctly use
mbox-cells = <2>. This commit fixes the example.

Fixes: 19d8e335

 ("dt-binding: gce: remove atomic_exec in mboxes property")
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Fabien Parent <fparent@baylibre.com>
Link: https://lore.kernel.org/r/20201018193016.3339045-1-fparent@baylibre.com
Signed-off-by: Rob Herring <robh@kernel.org>