samples: usb-audio: add app.overlay and use zephyr_udc0

USB audio class samples are generic and can be built for any
board that supports USB device and isochronous endpoints.
Add app.overlay that uses reference to zephyr_udc0 and remove
board specific overlays.

Note: USB audio is still experimental and even though the sample can
be built for specific platform, it does not mean that it can be run
on it without issues.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>
6 files changed