drivers: modem_cellular: add nRF91 series (w/ SLM)

The nRF91 series devices, when running the Serial LTE Modem (SLM)
application starting with nRF Connect SDK 2.6.0, can now be used as
standalone modems via the generic modem_cellular driver.

A configuration to run the cellular_modem sample on the nRF9160 DK
(plugged in to another nRF91 series device running SLM) is provided.

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