boards: nrf5340dk_nrf5340: Add dts node for MX25R64 flash on QSPI The nRF5340 (P)DK is equipped with the MX25R64 flash memory. Add a dts node for that chip in the board definition as well as the missing QSPI node in the nRF5340 SoC definition. Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>