tests/flash: Fix nrf52840dk configurations

The commit disables QSPI in SoC configurations, as QSPI got enabled
by default and test never really run on SoC.

In QSPI configuration erase page size is set to 4096
There is not TC_PRINT showing name of device that the test will
run on, in test setup.

The nrf52840dk configuration files have been renamed to reflect
dk name and SoC.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>