dt-bindings: sound: Add generic serial MIDI device

Adds dt-binding for a Generic MIDI Interface using a serial device.

Signed-off-by: Daniel Kaehn <kaehndan@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220509145933.1161526-2-kaehndan@gmail.com


Signed-off-by: Takashi Iwai <tiwai@suse.de>