boards: nrf: Use high drive for QSPI pins in nRF DK boards

nRF52840 and nRF5340 specifications recommend setting the GPIO drive
strength for QSPI pins to high drive to ensure stable operation.
Use that drive mode in nRF DK boards where the signal on these pins
can be degraded when for example a logic analyzer is connected to
QSPI lines.

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