ALSA: snd-powermac: mixers for PowerMac G4 AGP

Add mixer controls for PowerMac G4 AGP (Screamer).

This patch fixes the regression in the recent snd-powermac which
doesn't support some G3/G4 PowerMacs:
    http://lkml.org/lkml/2008/10/1/220



Signed-off-by: Risto Suominen <Risto.Suominen@gmail.com>
Tested-by: Mariusz Kozlowski <m.kozlowski@tuxland.pl>
Cc: stable@kernel.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>