PCI: iproc: Convert link check to raw PCI config accessors

The current iproc driver host bridge controller driver requires struct
pci_bus to be created in order to carry out PCI link checks with standard
PCI config space accessors.

This struct pci_bus dependency is fictitious and burdens the driver with
unneeded constraints (eg to use separate APIs to create and scan the root
bus).

Add PCI raw config space accessors to the iproc driver and remove the
fictitious struct pci_bus dependency.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Scott Branden <sbranden@broadcom.com>
Cc: Ray Jui <rjui@broadcom.com>
Cc: Jon Mason <jonmason@broadcom.com>