ALSA: hda/realtek - Add LED class support for micmute LED

ALSA: hda/realtek - Add LED class support for micmute LED

Currently DMIC controls micmute LED via "audio mute LED trigger".

However, unlike Dell and Lenovo platforms, HP platforms don't provide a
way to control micmute LED via ACPI, it's controlled by HDA codec
instead.

So let's register an LED class for micmute so other subsystems like DMIC
can facilitate the codec-controlled LED.

Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Link: https://lore.kernel.org/r/20200430135209.14703-1-kai.heng.feng@canonical.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>