mailbox: mediatek: add support for adsp mailbox controller

This patch is to for MediaTek ADSP IPC mailbox controller driver
It is used to send short messages between processors with adsp

Signed-off-by: Allen-KH Cheng <allen-kh.cheng@mediatek.com>
Reviewed-by: Tzung-Bi Shih <tzungbi@google.com>
Reviewed-by: YC Hung <yc.hung@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>