msm: pcie: support the case of no endpoint

The PCIe endpoint may not exist on APQ8084. Modify the PCIe bus
driver to support this situation.

Change-Id: I6465aa1a07839a4ef6019579587722ad7c27e0bb
Signed-off-by: Yan He <yanhe@codeaurora.org>
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>