Documentation/gpu: Update amdgpu documentation

7957ec80 ("drm/amdgpu: Add FRU sysfs nodes only if needed") moved
the documentation for some of the sysfs nodes to amdgpu_fru_eeprom.c.
Update the documentation accordingly.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>