msm: pcie: add PCIe bridge support

PCIe Endpoints can be connected to PCIe Root Complex via PCIe
bridges. Add the support in PCIe bus driver to configure and
manage PCIe bridge.

Change-Id: I31c3d8bed3be5550be8b6b304533d35bbbf42932
Signed-off-by: Tony Truong <truong@codeaurora.org>