boards: qemu: cortex_a53: add virtio-mmio configuration

Add `virtio-mmio` nodes that are retrieved from qemu internal
devicetree dump, to dts.

Also, enable the virtio-rng device on the command line.

Note: Ideally, it would be possible to dynamically generate the
command line by looking at the devicetree state,
but this PR is a fixed implementation to simplify it.
This will be addressed separately.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>