Documentation/gpu: include description of AMDGPU hardware structure

Documentation/gpu: include description of AMDGPU hardware structure

This describes in broad lines the how an AMD GPU is organized, in
terms of hardware blocks.

This is Alex' description from the "gpu block diagram" thread, edited to
fit as ReST.

Original text: https://www.spinics.net/lists/amd-gfx/msg71543.html

Originally-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Yann Dirson <ydirson@free.fr>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>