vexpress64: Set the DM_RNG property

vexpress64: Set the DM_RNG property

Enable the DM_RNG virtio random number generator driver in order to
consume entropy within U-Boot. This allows U-Boot to inject entropy to the
kernel via UEFI, so the kernel can use that early, for instance for
address layout randomisation, or when the kernel does not provide an
entropy driver itself.

Signed-off-by: Debbie Horsfall <debbie.horsfall@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>