ALSA: hda/realtek - Parse analog loopback paths more generically

Improve the parser of analog loopback paths and handle in a more
generic way.  The following changes are included in this patch:

- Instead of assuming direct connections between pins and
  the mixer widget, track the whole path between them.  This fixes
  some missing connections like ALC660.

- Introduce the path list for loopback paths like input and output
  path lists.  Currently it's not used for any real purposes, yet.

Signed-off-by: Takashi Iwai <tiwai@suse.de>