dt-bindings: pci: qcom: Document PCIe bindings for SM8150 SoC

Document the PCIe DT bindings for SM8150 SoC. The PCIe IP is similar to
the one used on SM8250.

Link: https://lore.kernel.org/r/20220326060810.1797516-2-bhupesh.sharma@linaro.org
Signed-off-by: Bhupesh Sharma <bhupesh.sharma@linaro.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rob Herring <robh@kernel.org>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>