staging: media: tegra-video: add syncpts for Tegra20 to struct tegra_vi

In preparation to implement Tegra20 parallel video capture, add a variable
to hold the required syncpt and document all the syncpt variables.

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>