pci: msm: Add support to handle ep driver requested link down

In case PCIe endpoint driver infers link down because of
completion timeout for config space access or if link fails
to enumerate, ep driver can request to handle the link down.
This allows to stream line the link down notification to ep
driver to initiate recovery operation.

Change-Id: I652e60e570c1e90c9f96d1c197e4b5501cd60a1e
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>