mailbox: mtk-cmdq-mailbox: fix module autoloading

Add MODULE_DEVICE_TABLE(), so this module could be properly autoloaded
based on the alias from of_device_id table.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>