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
   3647	    608	      0	   4255	   109f	drivers/virtio/virtio_mmio.o

File size after constify virtio_mmio_match.
   text	   data	    bss	    dec	    hex	filename
   4063	    192	      0	   4255	   109f	drivers/virtio/virtio_mmio.o
Signed-off-by: NArvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>