video: move tegra dc driver into own folder

Move tegra dc driver to tegra20 directory and also mention
T30 in description of the driver's config option.

Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
[agust: add commit description]
Signed-off-by: Anatolij Gustschin <agust@denx.de>