Documentation/gpu: Add amdgpu and dc glossary

Documentation/gpu: Add amdgpu and dc glossary

In the DC driver, we have multiple acronyms that are not obvious most of
the time; the same idea is valid for amdgpu. This commit introduces a DC
and amdgpu glossary in order to make it easier to navigate through our
driver.

Changes since V3:
 - Yann: Add new acronyms to amdgpu glossary
 - Daniel: Add link between dc and amdgpu glossary

Changes since V2:
 - Add MMHUB

Changes since V1:
 - Yann: Divide glossary based on driver context.
 - Alex: Make terms more consistent and update CPLIB
 - Add new acronyms to the glossary

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