media: aspeed: add comments and macro

Add comments to describe video-stat and 'struct aspeed_video'.
Add macro, ASPEED_VIDEO_V4L2_MIN_BUF_REQ, to describe the buffers
needed.

Signed-off-by: Jammy Huang <jammy_huang@aspeedtech.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil: drop docbook tags, not needed]