ALSA: snd-aoa: handle master-amp if present

Some machines have a master amp GPIO that needs to be toggled to
get sound output, in addition to speaker/headphone/line-out amps.
This makes snd-aoa handle it, if present in the device tree, thus
making snd-aoa be able to output sound on PowerMac3,6, which was
previously handled by snd-powermac which also doesn't use the
master amp GPIO.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>