Add GPS Bias Tee circuit

[mohamedboubaker]

The current version of the PCB has a GPS antenna issue. The GPS signal is not strong enough for the SIM808 to get a GPS fix.
Probable root cause: lack of DC bias on the antenna line to activate the active GPS Antenna.

DC signal in the antenna line should be introduced and turned on and off using a transistor (or other switch) connected to a GPIO on the MCU. It should be activated and deactivated by software to control power consumption of the Active antenna.

[mohamedboubaker]

This hardware bug can be fixed by manually hooking the active antenna amplifier circuit to the power line 3.6V.
It seems that the SIM808 antenna input is tolerant to DC offset.

However it is best to include the Bias Tee circuit.