What is the purpose of RC in series on FPGA output pin?
If the resistor value was anything other than either 50 ohm, 75 ohm, 100 ohm or 120 ohm I might give the answer that it was for general deglitching of an IO line. 

So, it's probably a 50 ohm data termination that doesn't screw up the DC levels of the data signals (because of the 1200 pF capacitor) but does significantly cut-down on the transmission-line reflections. I've also seen as 51 ohm in series with 100 pF (and I've used similar to fix some rogue RS485 problems): -



Picture taken from Here.