PCI: iproc: Add PAXCv2 related binding

Add new compatible string "brcm,iproc-pcie-paxc-v2" to the iProc PCIe
device tree binding document.  "brcm,iproc-pcie-paxc-v2" is for the second
generation of the Broadcom iProc PCIe PAXC host controller.

Update the binding document with more detailed description of each
compatible string and compatible SoCs.

Add description of optional property "msi-map", for use with MSI
controllers with sideband data.

Signed-off-by: Ray Jui <ray.jui@broadcom.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>