drm/tegra: Add driver documentation

Adds some driver documentation for Tegra. It provides a short overview
of the hardware and software architectures.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>