In the Cyclone® V Hard IP for PCI Express* User Guide version 1.5 and earlier, you might see the description on pld_clk: "You must drive this clock with coreclkout_hip.".
However, in the Cyclone® V Avalon® Streaming (Avalon-ST) Interface for PCIe* Solutions User Guide version 18.0, you might see the description on pld_clk: "You can drive this clock with coreclkout_hip. If you drive pld_clk with another clock source, it must be equal to or faster than coreclkout_hip, but cannot be faster than 250 MHz. Choose a clock source with a 0 ppm accuracy if pld_clk operates at the same frequency as coreclkout_hip.".
Yes, you can drive pld_clk with another clock source. Follow the description of pld_clk in the Cyclone® V Avalon® Streaming (Avalon-ST) Interface for PCIe* Solutions User Guide version 18.0.
This information will be updated in a future release of the Cyclone® V Hard IP for PCI Express* User Guide.