Proxy driver for GPS modules supported by the PX4 firmware

This PR adds a very simple proxy driver for GNSS modules already supported PX4/Firmware. The driver could be enabled via GPS_TYPE parameter. The driver is useful e.g. for UAVCAN GNSS receivers, but not limited  to them.

Note: this driver uses the same architectural approach as AP_Compass_PX4 and AP_Baro_PX4 do. In contrast to them, it uses a direct orb_subscribe() instead of a device file interfaces (there is no such thing in the current PX4 Firmware for GNSS devices).