mailbox: add MODULE_LICENSE() for mtk-cmdq-mailbox.c

Fix missing MODULE_LICENSE() in mtk-cmdq-mailbox.c:

WARNING: modpost: missing MODULE_LICENSE() in drivers/mailbox/mtk-cmdq-mailbox.o

Fixes: 623a6143 ("mailbox: mediatek: Add Mediatek CMDQ driver")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: <linux-mediatek@lists.infradead.org>
Cc: CK Hu <ck.hu@mediatek.com>
Cc: Houlong Wei <houlong.wei@mediatek.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>