sierra: XMM-specific features in XMM based modems (e.g. EM7345)

We create XMM-based modems like the EM7345 with XMM-specific features like the AT+XLCSLSR based GNSS support. E.g.:

  $ mmcli -m a --location-status
    ------------------------
    Location | capabilities: 3gpp-lac-ci, gps-raw, gps-nmea, agps-msa, agps-msb
             |      enabled: 3gpp-lac-ci
             |      signals: no
    ------------------------
    GPS      | refresh rate: 30 seconds

  $ sudo mmcli -m a --location-enable-gps-nmea
  successfully setup location gathering

  $ sudo mmcli -m a --location-get
    --------------------------
    3GPP |      operator code: 214
         |      operator name: 7
         | location area code: 0000
         | tracking area code: 6FFE
         |            cell id: 0465CD20
    --------------------------
    GPS  |               nmea: $GPGSA,A,1,,,,,,,,,,,,,,,*1E
         |                     $GNGSA,A,1,,,,,,,,,,,,,,,,1*1D
         |                     $GNRMC,235951.000,V,,,,,,,,,,N,V*20
         |                     $GNVTG,,T,,M,,N,,K,N*32
         |                     $GNGGA,235951.000,,,,,0,0,,,M,,M,,*5F

Fixes #25 (closed)