i.MX95 PCIes have two reference clock inputs: one from internal PLL,
the other from off chip crystal oscillator. Use extref clock name to be
onhalf of the reference clock comes from external crystal oscillator.

Add external reference clock mode support when extref clock is present.

Main change in v6:
- Refer to Krzysztof's comments, let i.MX95 PCIes has the "ref" clock
  since it is wired actually, and add one more optional "extref" clock
  for i.MX95 PCIes.

Main change in v5:
- Update the commit message of first patch refer to Bejorn's comments.
- Correct the typo error and update the description of property in the
  first patch.
https://lore.kernel.org/imx/20250915035348.3252353-1-hongxing.zhu@nxp.com/ (local)

Main change in v4:
- Add one more reference clock "extref" to be onhalf the reference clock
  that comes from external crystal oscillator.
https://lore.kernel.org/imx/20250626073804.3113757-1-hongxing.zhu@nxp.com/ (local)

Main change in v3:
- Update the logic check external reference clock mode is enabled or
  not in the driver codes.
https://lore.kernel.org/imx/20250620031350.674442-1-hongxing.zhu@nxp.com/ (local)

Main change in v2:
- Fix yamllint warning.
- Refine the driver codes.
https://lore.kernel.org/imx/20250619091004.338419-1-hongxing.zhu@nxp.com/ (local)

[PATCH v6 1/3] dt-bindings: PCI: dwc: Add one more reference clock
[PATCH v6 2/3] dt-bindings: pci-imx6: Add one more external reference
[PATCH v6 3/3] PCI: imx6: Add external reference clock mode support

Documentation/devicetree/bindings/pci/fsl,imx6q-pcie.yaml      |  3 +++
Documentation/devicetree/bindings/pci/snps,dw-pcie-common.yaml |  6 ++++++
drivers/pci/controller/dwc/pci-imx6.c                          | 20 +++++++++++++-------
3 files changed, 22 insertions(+), 7 deletions(-)