video: tegra20: add DSI controller driver

Adds support for both DSI outputs found on Tegra. Only very
minimal functionality is implemented, so advanced features
like ganged mode won't work. Driver is heavily based on
mainline Tegra DSI and re-uses much of its features.

Only T30 is supported for now but T20 support can be added
if any supported devices will be found.

Driver is wrapped as panel driver since Tegra DC driver supports
only panel drivers calls.

Tested-by: Andreas Westman Dorcsak <hedmoo@yahoo.com> # ASUS TF600T T30
Tested-by: Svyatoslav Ryhel <clamor95@gmail.com> # HTC One X T30
Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>