arm: dts: rockchip: rk3399: Move U-Boot specific bits to rk3399-u-boot

There's some bits in the U-Boot rk3399.dtsi that aren't yet in the
upstream Linux dtsi but are needed for early boot. This moves them
to the u-boot.dtsi to make it easier to sync the rest of rk3399.dtsi
with upstream.

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