dt-bindings: pci: qcom: Document PCIe bindings for SM8250 SoC

Document the PCIe DT bindings for SM8250 SoC. The PCIe IP is similar to
the one used on SDM845, hence just add the compatible.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>