On i.MX, the PCIe reference clock might come from either internal system
PLL or external clock source. Add the external reference clock source
for reference clock.

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.

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/

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/

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

[PATCH v5 1/3] dt-bindings: PCI: dwc: Add one more reference clock
[PATCH v5 2/3] dt-bindings: pci-imx6: Add external reference clock
[PATCH v5 3/3] PCI: imx6: Add external reference clock mode support

Documentation/devicetree/bindings/pci/fsl,imx6q-pcie.yaml      |  7 ++++++-
Documentation/devicetree/bindings/pci/snps,dw-pcie-common.yaml |  6 ++++++
drivers/pci/controller/dwc/pci-imx6.c                          | 20 +++++++++++++-------
3 files changed, 25 insertions(+), 8 deletions(-)