boards: add USB node aliases

USB devicetree nodes in Zephyr have different names,
mostly derived from the designations in data sheets.
We also have an USB audio sample which has a kind of
experimental approach to configure audio devices, that
are not real hardware, via devicetree.
Sample itself is generic and can be run on any board with
USB device controller and isochronous endpoints support.

Add usbd0 alias to specific USB node to allow generic
USB audio sample to be build on nRF and i.MX RT boards.

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