soc: mediatek: cmdq: define the instruction struct

Define an instruction structure for gce driver to append command.
This structure can make the client's code more readability.

Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>