PCI: qcom-ep: Disable Master AXI Clock when there is no PCIe traffic

The Master AXI clock can be disabled when it is not used i.e., when there
is no traffic on the PCIe bus. This helps to save power during idle state.

[bhelgaas: tidy and wrap comment]
Link: https://lore.kernel.org/r/20220914075350.7992-8-manivannan.sadhasivam@linaro.org


Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>