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 v3:
- Update the logic check external reference clock mode is enabled or
  not in driver codes.

Main change in v2:
- Fix yamllint warning.
- Refine the driver codes.

[PATCH v3 1/2] dt-binding: pci-imx6: Add external reference clock
[PATCH v3 2/2] PCI: imx6: Add external reference clock mode support

Documentation/devicetree/bindings/pci/fsl,imx6q-pcie.yaml |  7 ++++++-
drivers/pci/controller/dwc/pci-imx6.c                     | 20 +++++++++++++-------
2 files changed, 19 insertions(+), 8 deletions(-)