Merge tag 'u-boot-rockchip-20200501' of https://gitlab.denx.de/u-boot/custodians/u-boot-rockchip

- dts clean up to use -u-boot for px30, rk3399 boards
- dts sycn from upstream kernel for rk3328, rk3399
- add rockchip rng driver
- new board support: rk3328-roc-cc, rk3399-roc-pc,Nanopi M4 2GB