Documentation/gpu: include description of some of the GC microcontrollers

Documentation/gpu: include description of some of the GC microcontrollers

This is Alex' description from the "Looking for clarifications around gfx/kcq/kiq"
thread, edited to fit as ReST.

Original text: https://www.spinics.net/lists/amd-gfx/msg71383.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>