arm64: dts: rockchip: set to CCI clock of RK3399 to 600M

Per testing, this can reduce the memory latency and d8 gets
better scores.

Signed-off-by: Shunqian Zheng <zhengsq@rock-chips.com>
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>