dt-bindings: mailbox: mtk,adsp-mbox: Add compatible for MT8188

Add compatible string for ADSP mailbox on MT8188 SoC, which is
compatible with the one used on MT8186.

Acked-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Fei Shao <fshao@chromium.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>