platform/x86: thinkpad_acpi: Add audio mute LED classdev support

In the upcoming change, the binding of audio mute / mic-mute LED
controls will be switched with LED trigger.  This patch is the last
piece of preparation: adding the audio mute / mic-mute LED class
devices to thinkpad_acpi driver.

Two devices, platform::mute and platform::micmute, will be added for
controlling the mute LED and mic-mute LED, respectively.  The new
prefix "platform" is the suggestion by upstream for indicating the
generic laptop attribute.

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: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>