Overhaul clocking for XCKU MGTs

Overhaul clocking for XCKU MGTs

The clock inputs and outputs are now exposed to the instantiating layer.
This allows more flexible clocking schemes, including a completely
synchrounous system with multiple quads.

The reference-clock frequency is now set 100 MHz, so that it matches the
user-clock frequency.

The single-GT version is removed to simplify maintenance. Where needed,
it can be replaced by the quad version.