[ALSA] snd-powermac: AWACS and Screamer mixers for PM7500, Beige, and iMac SL

Add mixer controls and correct headphone detection bits for PowerMacs
7300/7500 (AWACS) and G3 Beige (Screamer), and iMac G3 Slot-loading
(Screamer).

Signed-off-by: Risto Suominen <Risto.Suominen@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>