media: mtk-vcodec: Add core dec and dec end ipi msg

Add core dec and dec end ipi msg:
AP_IPIMSG_DEC_CORE/AP_IPIMSG_DEC_CORE_END.

Signed-off-by: Yunfei Dong <yunfei.dong@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>