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>

Documentation/gpu/index.rst

@@ -12,6 +12,7 @@ Linux GPU Driver Developer's Guide
    drm-uapi
    i915
    meson
+   tegra
    tinydrm
    vc4
    vga-switcheroo