boards/arm/nrf52840_mdk_usb_dongle: enable the console driver

The nRF52840 MDK USB Dongle board selects the `uart0` node as
`zephyr,console` in the `chosen` node. This required three additional
options to be y-selected on Kconfig side so that printf/stdout etc. hooks
are properly installed:
* CONFIG_CONSOLE
* CONFIG_SERIAL
* CONFIG_UART_CONSOLE

Without that, no console output is observed.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>