virtio: virtio_mmio: 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
   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: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_mmio.c

@@ -723,7 +723,7 @@ static void vm_unregister_cmdline_devices(void)
 
 /* Platform driver */
 
-static struct of_device_id virtio_mmio_match[] = {
+static const struct of_device_id virtio_mmio_match[] = {
 	{ .compatible = "virtio,mmio", },
 	{},
 };