configs: rk3399: enable rng on firefly/rock960/rockpro64

Enable the RNG on the Firefly, rock960 and RockPro64 devices
to enable KASLR on devices that support it.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Cc: Akash Gajjar <akash@openedev.com>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Cc: Kever Yang <kever.yang@rock-chips.com>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Kever <Yang&lt;kever.yang@rock-chips.com>