arm: dts: rockchip: rk3399: enable rng at the SoC level

The rng is embedded in the SoC so enable it in the device tree
universally, the use of it can be controlled by enabling/disabling
at the device config level.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Cc: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Kever <Yang&lt;kever.yang@rock-chips.com>