[MTK] MT8168/MT8175 LPDDR4 如何降频到1600Mbps

1，reduce freq to 1600MHz in preloader (2 changes)
vendor/mediatek/proprietary/bootable/bootloader/preloader/platform/mt8168/src/drivers/inc/dramc_api.h 
-#define DUAL_FREQ_K  1
+#define DUAL_FREQ_K  0
#endif
#ifdef DRAM_SLT
#define CFG_DRAM_CALIB_OPTIMIZATION 0 /*slt bypass fast k*/
#else
-#define CFG_DRAM_CALIB_OPTIMIZATION (SUPPORT_TYPE_LPDDR4 | SUPPORT_TYPE_PCDDR4)
+#define CFG_DRAM_CALIB_OPTIMIZATION 0 //(SUPPORT_TYPE_LPDDR4 | SUPPORT_TYPE_PCDDR4)
#endif
2，disable Vcore DVFS in kernel (1 change)
kernel-4.14/arch/arm64/configs/XXXX_defconfig
-CONFIG_PM_DEVFREQ=y
+# CONFIG_PM_DEVFREQ is not set