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.

It's my fault missing add the necessary people and lists of dt-binding
review. Resend the v1 patch-set after correct dt-binding review pepople
and list. All the comments would be addressed, then send out v2 later.

[RESEND PATCH v1 1/2] dt-binding: pci-imx6: Add external reference
[RESEND PATCH v1 2/2] PCI: imx6: Add external reference clock mode

Documentation/devicetree/bindings/pci/fsl,imx6q-pcie.yaml |  7 ++++++-
drivers/pci/controller/dwc/pci-imx6.c                     | 34 ++++++++++++++++++++++++++--------
2 files changed, 32 insertions(+), 9 deletions(-)