Merge branch 'remotes/lorenzo/pci/xilinx-nwl'

- Document optional clock DT property (Michal Simek)

- Enable PCIe ref clock (Hyun Kwon)

* remotes/lorenzo/pci/xilinx-nwl:
  PCI: xilinx-nwl: Enable the clock through CCF
  dt-bindings: pci: xilinx-nwl: Document optional clock property