dt-bindings: PCI: fsl,imx6q: fix assigned-clocks warning

dt-bindings: PCI: fsl,imx6q: fix assigned-clocks warning

assigned-clocks are a dependency of clocks, however the dtschema has
limitation and expects clocks to be present in the binding using
assigned-clocks, not in other referenced bindings.  The clocks were
defined in common fsl,imx6q-pcie-common.yaml, which is referenced by
fsl,imx6q-pcie-ep.yaml.  The fsl,imx6q-pcie-ep.yaml used assigned-clocks
thus leading to warnings:

  Documentation/devicetree/bindings/pci/fsl,imx6q-pcie-ep.example.dtb: pcie-ep@33800000:
    Unevaluated properties are not allowed ('assigned-clock-parents', 'assigned-clock-rates', 'assigned-clocks' were unexpected)
  From schema: Documentation/devicetree/bindings/pci/fsl,imx6q-pcie-ep.yaml

Fix this by moving clocks to each specific schema from the common one
and narrowing them to strictly match what is expected for given device.

Fixes: b10f82380eeb ("dt-bindings: imx6q-pcie: Restruct i.MX PCIe schema")
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Richard Zhu <hongxing.zhu@nxp.com>
Link: https://lore.kernel.org/r/20230508071837.68552-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>