ALSA: ALS4000, slight mixer improvements

- add 8kHz / 20 kHz low-pass filter switch control
- add ALS4000 Mono capture route control
- add annotations to specs pages
- improve ALS4000 PM saved regs selection (remove SB dummy register,
  add missing ones)
- add some missing ALS4000 register defines
- constify two variables

Signed-off-by: Andreas Mohr <andi@lisas.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>