drm/amdgpu: add initial documentation for debugfs files

Describes what debugfs files are available and what
they are used for.

v2: fix some typos (Mark Glines)
v3: Address comments from Siqueira and Kent

Reviewed-by: Rodrigo Siqueira <siqueira@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>