tests: drivers: flash: Update nrf52840dk_mx25r_high_perf.overlay

This is a follow-up to commit 7a83724e.

This overlay uses an alternative connection (via spi2) for the external
flash present on the nRF52840 DK and it needs to use one of the QSPI
pins as GPIO, to get CS line control in the SPI communication. To make
it possible, that GPIO must be removed from those marked as reserved.

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