[ALSA] hda-codec - Add IEC958 digital out support for Lenovo Thinkpads T61/X61

This patch adds IEC958 digital out support for the AD1984 sound card.
This card can be found in Lenovo Thinkapds T61/X61. The digital out is
not located on the Thinkpad, but optional docking station (it's coxial
digital out). I've add this support as it is done the exact same way
for the AD1983 & AD1884.
I have tested this patch with my Lenovo Thinkpad T61 hooked up to a
docking station (that has the digital coxial) and then run to my home
theater reciever. Works like a charm :-)

Signed-off-by: Jerone Young <jerone@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>