boards: arm: nrf9160dk_nrf52840: flow control pins crossed

Devicetree pinout was written according to datasheet,
but RX and TX were crossed because labels in the datasheet
are written from debug interface point of view.
However, it seems that flow control pins were not crossed.

Fixes #54768

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>