dt-bindings: PCI: qcom: Document ddrss_sf_tbu clock for sm8250

On SM8250 additional clock is required for PCIe devices to access NOC.
Document this requirement in devicetree bindings.

Link: https://lore.kernel.org/r/20210117013114.441973-2-dmitry.baryshkov@linaro.org


Fixes: 45816824 ("dt-bindings: pci: qcom: Document PCIe bindings for SM8250 SoC")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>