You should be able to reclaim that memory by only modifying device tree but I cannot promise you that it won’t crash - if it does, you would need to deselect the drivers in kernel options and recompile the kernel.
By the way, what OS do you need to use? If Linux, perhaps you can try using an OS with mainline kernel which for starters is free of codec_mm CMA reservations? [VIM] Ubuntu Mate with Linux-4.14.11 and U-boot-v2018.01-rc3 ROM alpha-V180106 [SD/USB INSTALLATION]