ALSA: HDA: Enable SKU quirks for Realtek

Realtek have ways of specifying external amps and more via a
special nid or via the Codec's subsystem ID, this is called "SKU".
The computer manufacturer sometimes gets this wrong, so we need
to be able to override or ignore the SKU customization value.

Signed-off-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>