dt-bindings: Add QEMU virt machine compatible

The top level QEMU virt machine compatible, linux,dummy-virt, has been
in use for a long time, but never documented. Add a schema for it.
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20220310021224.599398-1-robh@kernel.org

Documentation/devicetree/bindings/arm/linux,dummy-virt.yaml

+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/arm/linux,dummy-virt.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: QEMU virt machine
+
+maintainers:
+  - Rob Herring <robh@kernel.org>
+
+properties:
+  $nodename:
+    const: "/"
+  compatible:
+    const: linux,dummy-virt
+
+additionalProperties: true
+
+...