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