firmware: mediatek: Add adsp ipc protocol interface

Some of mediatek processors contain
the Tensilica HiFix DSP for audio processing.

The communication between Host CPU and DSP firmware is
taking place using a shared memory area for message passing.

ADSP IPC protocol offers (send/recv) interfaces using
mediatek-mailbox APIs.

We use two mbox channels to implement a request-reply protocol.

Signed-off-by: Allen-KH Cheng <allen-kh.cheng@mediatek.com>
Signed-off-by: TingHan Shen <tinghan.shen@mediatek.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Curtis Malainey <cujomalainey@chromium.org>
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>
Link: https://lore.kernel.org/r/20220512082215.3018-2-tinghan.shen@mediatek.com
Signed-off-by: Mark Brown <broonie@kernel.org>