usb: Add USB audio implementation

By this commit USB audio class implementation is introduced
to Zephyr.

The Zephyr USB audio device class follows bellow
documentations:

- Universal Serial Bus specification rev2.0 (usb20.pdf)
- Universal Serial Bus Device Class Definition for Audio Devices
  (audio10.pdf)
- Universal Serial Bus Device Class Definition for Audio Data Formats
  (frmts10.pdf)
- Universal Serial Bus Device Class Definition for Terminal Types
  (termt10.pdf)

Signed-off-by: Emil Obalski <emil.obalski@nordicsemi.no>