This option refers only to the tegra20 video driver, so name it as such
to avoid confusion with tegra124.

Also move this option to Kconfig.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Tom Warren <twarren@nvidia.com>