dt-bindings: virtio: Convert virtio-mmio to DT schema

Convert the virtio-mmio binding to DT schema format.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Jean-Philippe Brucker <jean-philippe@linaro.org>
Cc: virtualization@lists.linux-foundation.org
Acked-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210607193928.3092186-1-robh@kernel.org/