samples: net: cellular_modem: add nRF9160 DK's nRF52840

The added configuration files allow the sample to be run
on the nRF52840 present on the nRF9160 DK with the nRF9160
(running SLM) being the modem and communication happening
through pins that are routed internally between the two.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>