Add parameter to select position of North/South Links

This branch adds a parameter `NORTH_SOUTH_ON_FRONT` which allows the user to select between the North and South links using the bottom two front sockets or the top two rear sockets.

This change is complicated by the fact that peripheral links operate at a different speed to internal links and so multiple GTP IPCOREs must be generated and switched between (using a generate) to cope with the difference until the dynamic reconfiguration port comes into use.