modemmanager: start a 3gpp network regsistration

modemmanager: start a 3gpp network regsistration

If a 'plmn' is set in the configuration, a registration attempt should be
established before the simple-connect command. If the plmn is set during
the simple-connect, a network change may occur during the connection setup.

To prevent this, the registration is started before the simple-connect with
a separate mmcli command. So that we can be sure that the modem is already
registered in the correct network before the simple-connect command.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>