RK3568平台（内存篇）DDR定频修改
一.DDR定频修改
项目背景：
在移植RK系统的时候系统一直会不断的重启，然后在打出来的log中看到两行异常的log：
[ 1.538750][ T1] rockchip-dmc dmc: Get wrong frequency, Request 1560000000, Current 1848000000
[ 1.538941][ T52] rockchip-dmc dmc: dev_pm_opp_set_rate: failed to find current OPP for freq 1848000000 (-34)ddr频点不对，先把ddr变频关掉，关闭后系统就不会一直重启了，以下是修改DDR频点的。
DDR 定频操作
rk3588修改定频节点在rk3588s.dtsi 的 dmc: dmc
dmc: dmc {
compatible = "rockchip,rk3588-dmc";
interrupts = <GIC_SPI 73 IRQ_TYPE_LEVEL_HIGH>;
interrupt-names = "complete";
devfreq-events = <&dfi>;
clocks = <&scmi_clk 4>;
clock-names = "dmc_clk";
operating-p