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>