ALSA: control - add sysfs support to the LED trigger module

Create SYSFS/devices/virtual/sound/ctl-led tree
(with SYSFS/class/sound/ctl-led symlink).

  speaker/
    +-- mode
    +-- brightness
  mic/
    +-- mode
    +-- brightness

Copy the idea from the HDA driver and allow to set the audio
LEDs based on the various modes:

- follow mute
- follow moute (inverted to follow mute)
- off
- on

Also, the actual LED state is exposed.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20210317172945.842280-6-perex@perex.cz


Signed-off-by: Takashi Iwai <tiwai@suse.de>