msm: pcie: support power transition by PCIe clients

PCIe devices may be disabled when the system is not in power
collapse. The change here provides PCIe clients with pm control
where they could indicate PCIe bus driver to suspend or resume
PCIe connection.

Change-Id: I1d70467cf736f711e97300b3267411e97300b296
Signed-off-by: Yan He <yanhe@codeaurora.org>