dt-bindings: virtio: mmio: add optional wakeup-source property

Some systems want to set the interrupt of virtio_mmio device
as a wakeup source. On such systems, we'll use the existence
of the "wakeup-source" property as a signal of requirement.

Signed-off-by: Minghao Xue <quic_mingxue@quicinc.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Message-Id: <1654851507-13891-2-git-send-email-quic_mingxue@quicinc.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>