drivers: Add video API

This generic video API can be used to capture/output video frames.

Once a video buffer is enqueued to a video device endpoint, device owns
the buffer and can process to capture (camera), output (disk, display),
convert (hw encoder)... User can then call dequeue to retrieve
the processed buffer (video driver ensure cache coherency).

Once dequeued, video buffer is owned by user (e.g. for frame
processing, display buffer update, write to media, etc...).

For each video-buffer, user needs allocate the associated frame buffer
via video_buffer_alloc. Buffer format is defined by video device
endpoint configuration. Video device can be controlled (e.g. contrast,
brightness, flip...) via controls.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>