board: arm: added new board nrf52840_mdk_usb_dongle

Origin: Original

Source code of the board/arm/nrf52840dongle_nrf52840
was adapted to be used for the nrf52840_mdk_usb_dongle board.
The main differences to the copied code are:
- Kconfig: deleted BOARD_ENABLE_DCDC_HV option
- Deleted all unnecessary modules from the device tree.

Signed-off-by: Nikola Trifunovic <nikola@trifunovic.at>