ALSA: usb-audio: Define USB MIDI 2.0 specs

Define new structs and constants from USB MIDI 2.0 specification,
to be used in the upcoming MIDI 2.0 support in USB-audio driver.

A new class-specific endpoint descriptor and group terminal block
descriptors are defined.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20230523075358.9672-9-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>