PCI: iproc: Add PAXBv2 binding info

Add new compatible string 'brcm,iproc-pcie-paxb-v2', for the next
generation of the iProc PAXB PCIe host controller.

Signed-off-by: Oza Oza <oza.oza@broadcom.com>
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>