Modem cellular API - registration status

[papadkostas]

As a user I would expect that I get current registration and status of RAT from get_registration_status API call.

Currently in modem cellular API, get_registration_status requires the application to input for which cellular access technology to get status for, so you have to itterate for every RAT until you get the one active, while a modem cannot concurently use different access technologies - if not mistaken. Additionally, modem_cellular_data holds status for each access technology separately when its not really needed, unless there is something more to keep track later per RAT.