drm/doc: Add initial amdgpu driver documentation

Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Signed-off-by: NMichel Dänzer <michel.daenzer@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

Documentation/gpu/amdgpu.rst

+=========================
+ drm/amdgpu AMDgpu driver
+=========================
+
+The drm/amdgpu driver supports all AMD Radeon GPUs based on the Graphics Core
+Next (GCN) architecture.

Documentation/gpu/drivers.rst

@@ -4,6 +4,7 @@ GPU Driver Documentation
 
 .. toctree::
 
+   amdgpu
    i915
    meson
    pl111