msm: pcie: add PCIe debugfs

This adds the pci_msm directory in debug with two entries:

 - rc_sel: specifies which RC(s) to test with.
 - case: specifies which case to run.

Change-Id: I77d5e11d73a1d7edc9a6464391b333937c48e040
Signed-off-by: Tony Truong <truong@codeaurora.org>