staging: media: tegra-video: improve documentation of tegra_video_format fields

Some fields are irrelevant for Tegra20/VIP. Add a note to clarify that.

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>