PCI: iproc: Update iProc PCIe device tree binding

Add a new compatible string "brcm,iproc-pcie-paxc", for PAXC-based iProc
PCIe root complex.  A PAXC-based PCIe root complex is connected to emulated
endpoint devices internal to the ASIC.

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