Draft: 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.