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

Documentation/devicetree/bindings/sound/serial-midi.yaml

+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/sound/serial-midi.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Generic Serial MIDI Interface
+
+maintainers:
+  - Daniel Kaehn <kaehndan@gmail.com>
+
+description:
+  Generic MIDI interface using a serial device. This denotes that a serial device is
+  dedicated to MIDI communication, either to an external MIDI device through a DIN5
+  or other connector, or to a known hardwired MIDI controller. This device must be a
+  child node of a serial node.
+
+  Can only be set to use standard baud rates corresponding to supported rates of the
+  parent serial device. If the standard MIDI baud of 31.25 kBaud is needed
+  (as would be the case if interfacing with arbitrary external MIDI devices),
+  configure the clocks of the parent serial device so that a requested baud of 38.4 kBaud
+  resuts in the standard MIDI baud rate, and set the 'current-speed' property to 38400 (default)
+
+properties:
+  compatible:
+    const: serial-midi
+
+  current-speed:
+    description: Baudrate to set the serial port to when this MIDI device is opened.
+    default: 38400
+
+required:
+  - compatible
+
+additionalProperties: false
+
+examples:
+  - |
+    serial {
+        midi {
+            compatible = "serial-midi";
+        };
+    };
+  - |
+    serial {
+        midi {
+            compatible = "serial-midi";
+            current-speed = <115200>;
+        };
+    };