XCKU MGTs: Change from quads to individual links

XCKU MGTs: Change from quads to individual links

This makes it easier to run the transceivers within a single quad as
individual links with separate line rates. Additional changes:

- Change from QPLL to CPLLs.
- Provide a single top entity for multiple possible
  frequency/reference-clock combinations.
- Remove GT reset logic and rely on the TrbNet reset logic. This is not
  fully compatible with GTH cores. In particular, RX PCS reset must be
  ignored. Otherwise, RX allow is asserted too early, and faulty data
  reaches the RX control state machine.
- Change the default equalizer mode to LPM, which is more reliable that
  DFE for 8b10b data with non-random sequences.
- Change the clock-correction sequences to 4 words:
      (K)BC (D)C5 (K)BC (D)50
  and (K)BC (D)50 (K)BC (D)50