msm: pcie: request L1 exit to clear pending TLP

Provide the API for PCIe client driver to request PCIe link to
exit L1 so that the pending write TLP could go out in time.

Change-Id: I0b850bb0896c859ada89b70e2b7facdcd84150da
Signed-off-by: Yan He <yanhe@codeaurora.org>