Adapt for PCIe x4

• Enable PCIe x4 by default
• Use the BSP constants to invert PN pairs on MGT clocks if necessary.

Rely on the application part of the firmware to disable I/O buffer insertion by setting pin constraints, e.g.:

set_property IO_BUFFER_TYPE NONE [get_ports {CPS_MGT_RX_P[0]}]
set_property IO_BUFFER_TYPE NONE [get_ports {CPS_MGT_RX_N[0]}]