PCI: qcom-ep: Expose link transition counts via debugfs

Qualcomm PCIe controllers have debug registers in the MMIO region
that count PCIe link transitions. Expose them over debugfs to
userspace to help debug the low power issues.

Link: https://lore.kernel.org/r/20220914075350.7992-6-manivannan.sadhasivam@linaro.org


Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>