boards: nrf9160dk_nrf52840: Add missing inclusion

This code uses the `NRF_DT_GPIOS_TO_PSEL` macro, so it should include
`<soc.h>` explicitly and not rely on the header being included by some
other one.

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