samples: usb: Fixes usb/console sample for nrf52840

Adds the virtualcom device to the nrf52840 SoC dts and adds a board
specific nrf52840_overlay.dts file, copied exactly from the
arduino101_overlay.dts file, to set the console device name
appropriately for the subsys/usb/console sample project.

There were no clear alternatives to add a usb_cdc: virtualcom device to
all known SoC devices with USB support as should probably happen.

There was also no clear alternatives to add a non-board specific overlay
to the example.

This fixes a specific problem with the nrf52840-dk board for this sample
project.

Signed-off-by: Tom Burdick <thomas.burdick@gmail.com>