Improve routing of FMC transceivers
While ibert test shows an open eye for FMC MGT 0..3 even for 12.5Gbps, the performance of MGT4 and MGT5 is much worse:
MGT4, 8Gbps, PRBS 15-bit:
![ibert_DIOT_SB_FMC_MGT4_8Gbps_PRBS15](data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==)
MGT5, 8Gbps, PRBS 15-bit:
![ibert_DIOT_SB_FMC_MGT5_8Gbps_PRBS15](data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==)
MGT4, 10Gbps, PRBS 15-bit:
![ibert_DIOT_SB_FMC_MGT4_10Gbps_PRBS15](data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==)
MGT5, 10Gbps, PRBS 15-bit:
![ibert_DIOT_SB_FMC_MGT5_10Gbps_PRBS15](data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==)