PCI: qcom: add runtime pm support to pcie_port

This patch is required when the pcie controller sits on a bus with
its own power domain and clocks which are controlled via a bus driver
like simple pm bus. As these bus driver have runtime pm enabled, it makes
sense to update the usage counter so that the runtime pm does not suspend
the clks or power domain associated with the bus driver.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>