msm: pcie: add global lock for PCIe enumeration

Multiple root complexes can enumerate concurrently.
Add a global lock to prevent race condition when PCI
framework discovers and updates the PCIe bus
driver's global device table.

Change-Id: I9c88f12aef9fce05d95cfb2fa0f5374c6c4ab1e8
Signed-off-by: Tony Truong <truong@codeaurora.org>