msm: pcie: add mutex for PCIe enumeration

PCIe bus driver supports a few ways for clients to
trigger enumeration. To prevent race conditions,
add a mutex to secure the enumeration process.

Change-Id: Ifa4f6d2b4edee85e750e6e44626c89ec0de966cf
Signed-off-by: Tony Truong <truong@codeaurora.org>