remoteproc: mediatek: Add IMGSYS IPI command

Add an IPI command definition for communication with IMGSYS through
SCP mailbox.

Signed-off-by: Olivia Wen <olivia.wen@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20240430011534.9587-5-olivia.wen@mediatek.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>