feat: add MGT ref clocks from the Main PLL
The "Main PLL" provides clocks to the Multi-Gigabit Transceivers (MGTs) that are used for the FMC GBT links. This change assigns their pins to input buffer instances (IBUFDS_GTE4).
The clock buffer has two outputs (normal and divided). They are routed to the payload, so they can be used by an application that is implementing MGTs for FMC interfaces.
Resolves #1 (closed)
Edited by Michael Buechler