ALSA: Add documentation about HD-audio DP MST

Add the documentation about HD-audio DP MST:
1. Overview
2. Jack

Others will be added later.
Signed-off-by: Libin Yang <libin.yang@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/alsa/HD-Audio-DP-MST-audio.txt

+To support DP MST audio, HD Audio hdmi codec driver introduces virtual pin
+and dynamic pcm assignment.
+
+Virtual pin is an extension of per_pin. The most difference of DP MST
+from legacy is that DP MST introduces device entry. Each pin can contain
+several device entries. Each device entry behaves as a pin.
+
+As each pin may contain several device entries and each codec may contain
+several pins, if we use one pcm per per_pin, there will be many PCMs.
+The new solution is to create a few PCMs and to dynamically bind pcm to
+per_pin. Driver uses spec->dyn_pcm_assign flag to indicate whether to use
+the new solution.
+
+PCM
+===
+To be added
+
+
+Jack
+====
+
+Presume:
+ - MST must be dyn_pcm_assign, and it is acomp (for Intel scenario);
+ - NON-MST may or may not be dyn_pcm_assign, it can be acomp or !acomp;
+
+So there are the following scenarios:
+ a. MST (&& dyn_pcm_assign && acomp)
+ b. NON-MST && dyn_pcm_assign && acomp
+ c. NON-MST && !dyn_pcm_assign && !acomp
+
+Below discussion will ignore MST and NON-MST difference as it doesn't
+impact on jack handling too much.
+
+Driver uses struct hdmi_pcm pcm[] array in hdmi_spec and snd_jack is
+a member of hdmi_pcm. Each pin has one struct hdmi_pcm * pcm pointer.
+
+For !dyn_pcm_assign, per_pin->pcm will assigned to spec->pcm[n] statically.
+
+For dyn_pcm_assign, per_pin->pcm will assigned to spec->pcm[n]
+when monitor is hotplugged.
+
+
+Build Jack
+----------
+
+- dyn_pcm_assign
+Will not use hda_jack but use snd_jack in spec->pcm_rec[pcm_idx].jack directly.
+
+- !dyn_pcm_assign
+Use hda_jack and assign spec->pcm_rec[pcm_idx].jack = jack->jack statically.
+
+
+Unsolicited Event Enabling
+--------------------------
+Enable unsolicited event if !acomp.
+
+
+Monitor Hotplug Event Handling
+------------------------------
+- acomp
+pin_eld_notify() -> check_presence_and_report() -> hdmi_present_sense() ->
+sync_eld_via_acomp().
+Use directly snd_jack_report() on spec->pcm_rec[pcm_idx].jack for
+both dyn_pcm_assign and !dyn_pcm_assign
+
+- !acomp
+Hdmi_unsol_event() -> hdmi_intrinsic_event() -> check_presence_and_report() ->
+hdmi_present_sense() -> hdmi_prepsent_sense_via_verbs()
+Use directly snd_jack_report() on spec->pcm_rec[pcm_idx].jack for dyn_pcm_assign.
+Use hda_jack mechanism to handle jack events.
+
+
+Others to be added later
+========================