hw/arm/virt: Fix devicetree warning about the root node

hw/arm/virt: Fix devicetree warning about the root node

The devicetree specification requires a 'model' property in the root
node. Fix the corresponding dt-validate warning:

  /: 'model' is a required property
  From schema: dtschema/schemas/root-node.yaml

Use the same name for model as for compatible. The specification
recommends that 'compatible' follows the format 'manufacturer,model' and
'model' follows the format 'manufacturer,model-number'. Since our
'compatible' doesn't observe this, 'model' doesn't really need to
either.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Message-id: 20220927100347.176606-2-jean-philippe@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>