Add support for GPS related commands

The SIM808_GPS_Application_Note_V1.00.pdf defines commands that can be used to obtain the longitude, latitude and datetime from built in GPS hardware. Would be good to have that command feature in the driver. Will do a pull request for that in the coming weeks.
AT+GPSPWR
AT+CGPSRST
AT+CGPSINF
AT+CGPSSTATUS