ALSA: oxfw: add scs1x layer

Stanton Controllers and Systems 1 (SCS.1) series is supported by ALSA
scs1x driver. This driver just supports MIDI functionality. On the other
hand, models in this series are based on OXFW971 and ALSA OXFW driver can
support them.

SCS.1 series has MIDI functionality to control its surface state such as
LED lighting. When operating physical knobs and faders, the models
generate MIDI messages. These MIDI messages are transferred by asynchronous
transactions. These transactions are really model-specific and ALSA OXFW
driver requires the functionality so as scs1x module implements.

This commit adds scs1x layer as a preparation to merge scs1x driver to
oxfw driver.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>