igc: Add legacy power management support

Add suspend, resume, runtime_suspend, runtime_resume and
runtime_idle callbacks implementation.

Reported-by: kbuild test robot <lpk@intel.com>
Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>