ALSA: hda - Allow to enable/disable vmaster build explicitly

Another preliminary patch for the dual-codec support: since the
support of vmaster over multiple codecs is difficult, simply disable
it by a new flag to hda_codec struct.  A new user hint is added as
well for consistency.

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

Documentation/sound/hd-audio/notes.rst

@@ -494,6 +494,8 @@ add_hp_mic (bool)
 hp_mic_detect (bool)
     enable/disable the hp/mic shared input for a single built-in mic
     case; default true
+vmaster (bool)
+    enable/disable the virtual Master control; default true
 mixer_nid (int)
     specifies the widget NID of the analog-loopback mixer