boards: bl5340_dvk: always enable NORDIC_QSPI_NOR

Change the NORDIC_QSPI_NOR default override to always be active on the
board. The current setup is breaking CI on USB DFU samples as those are
looking for some flash partitions on the external flash (see
bl5340_dvk_cpuapp_common.dts).

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>