drivers: video: Introduce video device structure

Introduce a new video device structure representing a device in a
video pipeline. Each video device embeds a pointer to its "source"
device and other "video" characteristics.

This structure give the video framework an access to all video features
of the device and a hierachical view of the video pipeline that the
device belongs to.

Signed-off-by: Phi Bang Nguyen <phibang.nguyen@nxp.com>