samples: nrfx_prs: Fix pin assignment on nRF5340 DK

Pins P0.06 and P0.07 are shorted on nRF5340 DKs prepared for on-target
tests because that is required for UART tests. That causes background
SPIM transfers in this sample to fail because SPIM is not able to drive
its SCK line configured as P0.06. Rearrange then the pins used in this
sample, so that SCK is not on a pin involved in a GPIO loopback.

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