dt-bindings: pci: qcom: Document PCIe bindings for IPQ6018 SoC

Document qcom,pcie-ipq6018. This is similar to the ipq8074 with a few
different clock sources, and one additional reset.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Link: https://lore.kernel.org/r/fd732635f4ad64263e361ce98af2944bfbd513ef.1620203062.git.baruch@tkos.co.il


Signed-off-by: Vinod Koul <vkoul@kernel.org>