* Add usbd_dc_nrfx shim
The shim is based on the previous one usbd_dc_nrf5.
For handling the USBD hardware, tested nrfx_usbd driver from nRF SDK
was used.
Briefly tested examples:
* usb/cdc_acm
* usb/dfu (USB communication only due to flash handling issues)
* usb/hid-mouse
* bluetooth/hci_usb
Signed-off-by: Paweł Zadrożniak <pawel.zadrozniak@nordicsemi.no>