Add MGT loopback for OH trigger links
It is known that, on GE1/1, the trigger links might end up being not properly initialized. No solution has been found until now, but it was noticed that when the link wasn't initialized properly, the data received by the RX part of the MGT in loopback mode was also garbage (8b10b symbol errors,...)
In order to understand if this is working at large scale, the MGT RX should be connected in loopback mode for all trigger links MGT. Errors should be counted so that the software can reset the MGT. If the solution is proven to work, firmware implementation can follow.