drivers: usbd_nrf5: Add nRF52840 USB Device Controller Driver

* Add USB device controller driver for nRF52840 SoC.
* Supports only Bulk and Interrupt endpoints for now.
* ISO endpoint support is WIP.

What's tested:
* BTUSB - LE Scanning and connection.

Known issues in btusb tests:
 * Observed momentary delays during LE scanning. This is possibly
due to hci_usb's way of calling the usb_write().

 * Sometimes, when LE scan stop command is issued, response is not
sent and as a result USB stops functioning. This is also related
to the way btusb is handling the write(). For some reason, the
btusb class handler seems to issue another data packet write before
the USB control transaction is complete.

Miscellaneous:
* Lots of TODO items to discuss and fix.

Origin: Original

Signed-off-by: Sundar Subramaniyan <sundar.subramaniyan@gmail.com>