Add kernel-doc for qcom_pcie_ep structure.

Link: https://lore.kernel.org/r/20220914075350.7992-2-manivannan.sadhasivam@linaro.orgSigned-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>