usb: device_next: add new MIDI 2.0 device class

This adds a new USB device class (based on usb/device_next) that implements
revision 2.0 of the MIDIStreaming interface, a sub-class of the USB audio
device class. In practice, the MIDI interface is much more simple and has
little in common with Audio, so it makes sense to have it as a separate
class driver.

MIDI inputs and outputs are configured through the device tree, under a
node `compatible = "zephyr,usb-midi"`. As per the USB-MIDI2.0 spec,
a single usb-midi interface can convey up to 16 Universal MIDI groups,
comprising 16 channels each. Data is carried from/to the host via
so-called Group Terminals, that are organized in Group Terminal Blocks.
They are represented as children of the usb-midi interface in the device
tree.

From the Zephyr application programmer perspective, MIDI data is exchanged
with the host through the device associated with the `zephyr,usb-midi`
interface, using the following API:

* Send a Universal MIDI Packet to the host: `usb_midi_send(device, pkt)`
* Universal MIDI Packets from the host are delivered to the function passed
  in `usb_midi_set_ops(device, &{.rx_packet_cb = handler})`

Compliant USB-MIDI 2.0 devices are required to expose a USB-MIDI1.0
interface as alt setting 0, and the 2.0 interface on alt setting 1.
To avoid the extra complexity of generating backward compatible USB
descriptors and translating Universal MIDI Packets from/to the old
USB-MIDI1.0 format, this driver generates an empty MIDI1.0 interface
(without any input/output); and therefore will only be able to exchange
MIDI data when the host has explicitely enabled MIDI2.0 (alt setting 1).

This implementation is based on the following documents, which are referred
to in the inline comments:

* `midi20`:
    Universal Serial Bus Device Class Definition for MIDI Devices
    Release 2.0
    https://www.usb.org/sites/default/files/USB%20MIDI%20v2_0.pdf
* `ump112`:
    Universal MIDI Packet (UMP) Format and MIDI 2.0 Protocol
      With MIDI 1.0 Protocol in UMP Format
    Document Version 1.1.2
    https://midi.org/universal-midi-packet-ump-and-midi-2-0-protocol-specification



Signed-off-by: Titouan Christophe <moiandme@gmail.com>