msm: pcie: enable client to control config space shadowing

PCIe client driver has the requirement to turn on and turn off
the shadowing of config space in PCIe bus driver to resume the
PCIe device or recover the link after expected linkdown happens.
Add the support in PCIe bus driver for this.

Change-Id: I7bc3d08fce33a6564b24dab1fa9f9591822e2f38
Signed-off-by: Yan He <yanhe@codeaurora.org>