samples: boards: nrf: Add sample for the display_nrf_led_matrix driver

Add a sample that can be used to see the nRF LED matrix display driver
in action and that will prove that the driver is buildable for both the
bbc_microbit_v2 and bbc_microbit boards.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>