dt-bindings: virtio,mmio: Allow setting devices 'dma-coherent'

'virtio,mmio' devices are always coherent within a guest. As Arm DTs
are default non-coherent, the 'dma-coherent' property needs to be set on
the virtio device to mark them as such. This aligns with the QEMU 'virt'
machine DTB.
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220310013528.549030-1-robh@kernel.org

Documentation/devicetree/bindings/virtio/mmio.yaml

@@ -20,6 +20,8 @@ properties:
   reg:
     maxItems: 1
 
+  dma-coherent: true
+
   interrupts:
     maxItems: 1