ALSA: hda/realtek - Fix speaker support for Asus AiO ZN270IE

Asus AiO ZN270IE with ALC256 has no audio ouput for internal speaker
and headphone. It requires GPIO 2 as an amp. This commit enables the
GPIO and pulls it high.

Signed-off-by: Chris Chiu <chiu@endlessm.com>
Signed-off-by: Daniel Drake <drake@endlessm.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>