arm64: dts: rockchip: add core dtsi for RK3568 SoC

RK3568 is a high-performance and low power quad-core application processor
designed for personal mobile internet device and AIoT equipment. This patch
add basic core dtsi file for it.

We use scmi_clk for cortex-a55 instead of standard ARMCLK, so that
kernel/uboot/rtos can change cpu clk with the same code in ATF, and we will
enalbe a special high-performance PLL when high frequency is required. The
smci_clk code is in ATF, and clkid for cpu is 0, as below:

    cpu0: cpu@0 {
        device_type = "cpu";
        compatible = "arm,cortex-a55";
        reg = <0x0 0x0>;
        clocks = <&scmi_clk 0>;
    };
Signed-off-by: Liang Chen <cl@rock-chips.com>
Link: https://lore.kernel.org/r/20210622020517.13100-4-cl@rock-chips.comSigned-off-by: Heiko Stuebner <heiko@sntech.de>