dell: implement Unmanaged GPS support for the DW5821E

The DW5821E module is managed in MBIM mode by default, and exposes a
NMEA capable tty in USB interface #4.

Enabling/disabling the NMEA output via the TTY is done with AT
commands, so this implementation requires also a valid AT port in the
system.

Given that the AT commands used to enable/disable this feature are
based on modifying non-volatile memory through AT^NV, this
implementation is very specific to the DW5821E. If we're able to do
the same on other Dell modules in the future, we'll just rename the
new object to a more generic one.