fix xilinx platform module to select between available GTP channels
In our officially supported boards we always used channel 1 GTP in the
Xilinx Spartan-6 FPGA. This might not always be the case, especially if
somebody wants to run the WR PTP Core on a custom hardware.
To fix this we have exported a selection VHDL generic from the
xwrc_platform_xilinx HDL module:
ae0028ff: platform/xilinx: add generics to Xilinx platform to select
which GTP channel to use (defaults to ch1)