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>

Documentation/devicetree/bindings/pci/brcm,iproc-pcie.txt

 * Broadcom iProc PCIe controller with the platform bus interface
 
 Required properties:
-- compatible: Must be "brcm,iproc-pcie"
+- compatible: Must be "brcm,iproc-pcie" for PAXB, or "brcm,iproc-pcie-paxc"
+  for PAXC.  PAXB-based root complex is used for external endpoint devices.
+  PAXC-based root complex is connected to emulated endpoint devices
+  internal to the ASIC
 - reg: base address and length of the PCIe controller I/O register space
 - #interrupt-cells: set to <1>
 - interrupt-map-mask and interrupt-map, standard PCI properties to define the