ALSA: hda - Add the generic Headphone Mic feature

This patch improves the generic parser code to allow to set up the
headphone jack as a mic input.  User can enable this feature by giving
hp_mic hint string.

The former shared hp/mic feature for the single built-in mic is still
retained.  This detection can be disabled now via hp_mic_detect hint
string, too.

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

Documentation/sound/alsa/HD-Audio.txt

@@ -466,6 +466,9 @@ The generic parser supports the following hints:
 - add_in_jack_modes (bool): add "xxx Jack Mode" enum controls to each
   input jack for allowing to change the mic bias vref
 - power_down_unused (bool): power down the unused widgets
+- add_hp_mic (bool): add the headphone to capture source if possible
+- hp_mic_detect (bool): enable/disable the hp/mic shared input for a
+  single built-in mic case; default true
 - mixer_nid (int): specifies the widget NID of the analog-loopback
   mixer