rockchip: px30: enable RNG for all boards

rockchip: px30: enable RNG for all boards

I don't see a reason why this should only be enabled on a per-board
basis. The rng IP is inside the SoC and doesn't seem to rely on anything
external to it, therefore let's enable it on the SoC DTSI and remove the
now empty px30-evb-u-boot.dtsi.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>