dt-bindings: mailbox: mediatek,gce-mailbox: add mt8188 compatible name

Add mt8188 compatible name.

Signed-off-by: Elvis Wang <Elvis.Wang@mediatek.com>
Reviewed-by: AngeloGioacchino Del <Regno&lt;angelogioacchino.delregno@collabora.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>