arm64: dts: rockchip: add core dtsi file for RK3399 SoCs

This patch adds core dtsi file for Rockchip RK3399 SoCs.

The RK3399 has big/little architecture, which needs a separate
node for the PMU of each microarchitecture, for now it missing
the pmu node since the old one could not work well.

Signed-off-by: Jianqun Xu <jay.xu@rock-chips.com>
Tested-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>