Add support for GPS-only modems (no cell functionality)

This is a split off of this comment from !1080 (merged). While MM now supports using the location interface of a cell modem without an active SIM card, it doesn't support initializing the location interface for "modems" that don't have any cell functionality at all and are just GPS devices. All the APQ8026 devices (list) are like this. An alternative approach of using gpsd isn't upstreamed and has no path to being upstreamed (see this issue).

I'm opening this bug as a reference as this comes up in the postmarketOS community occasionally, but also to decide whether this support should be in MM or somewhere else.

If this was supported, we'd need to audit the .Modem interface to see what properties were relevant (eg, SIM slot stuff probably won't be) and what weren't. We'd also likely need to define a MMModemCapability for GPS-only or something like that. Then obviously it wouldn't implement any of the interfaces other than Location.

It seems fairly do-able. The careful part needs to be how existing clients handle these devices, making sure they don't fail hard if the device doesn't have properties/interfaces they are expecting.