Documentation/gpu: Add more information about GC

Documentation/gpu: Add more information about GC

This commit introduces set of information that details the different
sets of schedulers available in the SE.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Rodrigo Siqueira <siqueira@igalia.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>