Documentation/gpu: How to collect DTN log

Introduce how to collect DTN log from debugfs.

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>