drm/amdgpu/pm: update hwmon power documentation

Power reporting is socket power.  On APUs this includes
the CPU.  Update the documentation to clarify this.

Reviewed-by: Aaron Liu <aaron.liu@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>