Initial files with license agreement

# gtxe2_nosecureip

GTXE2_CHANNEL_GPL.v is a non-synthesizable open-source replacement for a propriate xilinx's primitive GTXE2_CHANNEL.v

For now only basic features are implemented : OOB, comma aligment, 10-8 encoding/decoding, clocking scheme. 

Still, that is enough to run SATA protocol implementation simulation.