Add support for the Broadcom iProc PCIe controller.

pcie-iproc.c is the common core driver, and a front-end bus interface needs
to be added to support different bus interfaces.

pcie-iproc-platform.c contains the support for the platform bus interface.
Signed-off-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>