Lenovo Thinkpads with Realtek codecs may still have some loud
crackling noises at reboot/shutdown even though a few previous fixes
have been applied.  It's because the previous fix (disabling the
default shutup callback) takes effect only at transition of the codec
power state.  Meanwhile, at reboot or shutdown, we don't take down the
codec power as default, thus it triggers the same problem unless the
codec is powered down casually by runtime PM.

This patch tries to address the issue.  It gives two things:
- implement the separate reboot_notify hook to struct alc_spec, and
  call it optionally if defined.
- turn off the codec to D3 for Thinkpad models via this new callback

Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=958439


Reported-and-tested-by: Benjamin Poirier <bpoirier@suse.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>