PCI: iproc: Add PAXC interface support

Traditionally, all iProc PCIe root complexes use PAXB-based wrapper, with
an integrated on-chip Serdes to support external endpoint devices.  On
newer iProc platforms, a PAXC-based wrapper is introduced, for connection
with internally emulated PCIe endpoint devices in the ASIC.

Add support for PAXC-based iProc PCIe root complex in the iProc PCIe core
driver.  This change factors out common logic between PAXB and PAXC, and
uses tables to store register offsets that are different between PAXB and
PAXC.  This allows the driver to be scaled to support subsequent PAXC
revisions in the future.

Signed-off-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>