platform/x86: dell-laptop: Add micmute LED trigger support

This patch adds the LED trigger support for audio mic-mute control.
As of this patch, the LED device isn't tied with the audio driver, and
can be changed via user-space at "platform::micmute" sysfs entry.
(This new prefix "platform" is the agreement among people for
indicating the generic laptop / system-wide attribute.)

The binding with HD-audio is still done via the existing exported
dell_micmute_led_set().  It will be replaced with the LED trigger
binding in later patches.

Also this selects CONFIG_LEDS_TRIGGERS and CONFIG_LEDS_TRIGGERS_AUDIO
unconditionally.  Strictly speaking, these aren't 100% mandatory, but
leaving these manual selections would lead to a functional regression
easily once after converting from the dynamic symbol binding to the
LEDs trigger in a later patch.

Acked-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>