ALSA: revo51: add headphone output

It is ALSA bug #1754.
 D6 bit in 02 register is PW4.

Signed-off-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>