ALSA: Documentation about HDA DP MST pin init and connection

Add the documentation about HD-audio DP MST:
1. pin initialization
2. device entry connection list

Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Libin Yang <libin.yang@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1484208294-8637-4-git-send-email-libin.yang@intel.com