PCI: xilinx: Make of_device_ids const

of_device_ids are not supposed to change at runtime.  All functions working
with of_device_ids provided by <linux/of.h> work with const of_device_ids.
So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
    195	    600	      0	    795	    31b	drivers/pci/host/pcie-xilinx.o

File size after constify xilinx_pcie_of_match:
   text	   data	    bss	    dec	    hex	filename
    595	    184	      0	    779	    30b	drivers/pci/host/pcie-xilinx.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>