ALSA: hda: generic: Add a helper for mic-mute LED with LED classdev

ALSA: hda: generic: Add a helper for mic-mute LED with LED classdev

A new helper, snd_hda_gen_add_micmute_led_cdev(), is introduced here
for creating a LED classdev and setting up the hook to the capture
control for controlling the mic-mute LED to follow the capture switch
change.  This will replace the existing users of
snd_hda_gen_add_micmute_led() in later patches.

Also, introduce a new kconfig CONFIG_SND_HDA_GENERIC_LEDS, to indicate
the usage of mute / mic-mute LED helpers.  It's selected by the codec
drivers (Realtek, Conexant and Sigmatel), while it selects the
necessary LED class dependencies.

Tested-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Link: https://lore.kernel.org/r/20200618110842.27238-3-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>