staging: media: tegra-video: Kconfig: allow TPG only on Tegra210

We are about to add support for the Tegra20 parallel video capture, which
has no TPG. In preparation for that, limit the VIDEO_TEGRA_TPG option to
Tegra210 which is the only implementation currently provided by this
driver.

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>