dt-bindings: phy: qcom,qmp: Document SM8250 PCIe PHY bindings

Document the SM8250 PCIe PHY DT bindings. The PHY IPs (QMP, QHP) are
similar to the ones used on SDM845 hence just add the compatibles

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