microblaze/PCI: Support generic Xilinx AXI PCIe Host Bridge IP driver

Modify the Microblaze PCI subsystem to work with the generic
drivers/pci/host/pcie-xilinx.c driver on Microblaze and Zynq.

[bhelgaas: changelog]
Signed-off-by: Bharat Kumar Gogada <bharatku@xilinx.com>
Signed-off-by: Ravi Kiran Gummaluri <rgummal@xilinx.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>