MAINTAINERS: Add file patterns for virtio device tree bindings

Submitters of device tree binding documentation may forget to CC
the subsystem maintainer if this is missing.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

MAINTAINERS

@@ -12131,6 +12131,7 @@ VIRTIO CORE, NET AND BLOCK DRIVERS
 M:	"Michael S. Tsirkin" <mst@redhat.com>
 L:	virtualization@lists.linux-foundation.org
 S:	Maintained
+F:	Documentation/devicetree/bindings/virtio/
 F:	drivers/virtio/
 F:	tools/virtio/
 F:	drivers/net/virtio_net.c