fdts: Remove "virtio-rng" from Morello FVP

fdts: Remove "virtio-rng" from Morello FVP

This is not a standard string that any kernel recognises, nor do any of
the FDTs embedded in kernels specify this, nor does QEMU's virt machine.
Whilst its presence does no harm, it's not a thing code should consult
as a result, and so drop it in order to not cause confusion and risk
incorrect code being written to search for it.

Signed-off-by: Jessica Clarke <jrtc27@jrtc27.com>
Change-Id: Iea3214a23181c54e600cf8f4f12dfc822140c23d