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 v2:
- Fix yamllint warning.
- Refine the driver codes.

[PATCH v2 1/2] dt-binding: pci-imx6: Add external reference clock
[PATCH v2 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                     | 22 +++++++++++++++-------
2 files changed, 21 insertions(+), 8 deletions(-)