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