ALSA: hda/realtek - Parse input paths

Just like the output paths, parse the whole paths for inputs as well
and store in a path list.  For that purpose, rewrite the output parser
code to be generically usable.

The input path list is not referred at all in this patch.  It'll be
used to replace the fixed adc/capsrc array in later patches for more
flexible input path selections.

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