dt-bindings: PCI: imx6: convert the imx pcie controller to dtschema

Convert the fsl,imx6q-pcie.txt into a schema.
- ranges property should be grouped by region, with no functional
  changes.
- only one propert is allowed in the compatible string, remove
  "snps,dw-pcie".

Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Link: https://lore.kernel.org/r/1630046580-19282-2-git-send-email-hongxing.zhu@nxp.com


Signed-off-by: Rob Herring <robh@kernel.org>