msm: pcie: allow clients to run debug testcases

This change enhances client's ablilty to debug. For example,
PCIe link can go down if an error occurs. Clients can then use
PCIe API to run debug testcases before link goes down.

Change-Id: I6dbb9a991063b22684f07f3b9fbb91985cb7c329
Signed-off-by: Tony Truong <truong@codeaurora.org>