boards: arm: nrf5340: rename nRF5340 board definition directory

We rename the nRF5340 board directory to
nrf5340dk_nrf5340 since this will be now
containing board definitions for both the
nRF5340 PDK and DK.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>