Documentation/amdgpu: Add documentation for Peak Tops Limiter (PTL) sysfs interface

Documentation/amdgpu: Add documentation for Peak Tops Limiter (PTL) sysfs interface

The PTL (Peak Tops Limiter) feature exposes per-GPU sysfs files under
/sys/class/drm/cardX/device/ptl/ to allow users to enable or disable PTL,
configure preferred data formats, and query supported formats. The usage
of these sysfs files is not always obvious, so add documentation to
describe their purpose and provide concrete usage examples.

V3 changes:
 * format show will display preferred formats instead of N/A (Alex)

Signed-off-by: Perry Yuan <perry.yuan@amd.com>
Suggested-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Yifan Zhang <yifan1.zhang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>