drm/amdgpu/pm: document pp_power_profile_mode

sysfs file for adjusting power level heuristics.

Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>