you have to modify ahci driver and build the kernel.
Get the kernel here: https://github.com/BPI-SINOVOIP/BPI-M2U-bsp
Modify driver as described here: https://forum.armbian.com/topic/5199-banana-pi-m2u-wont-boot/?tab=comments#comment-39587 (thanks to Armbian developers)
Follow instructions how to build and install the kernel here: https://github.com/BPI-SINOVOIP/BPI-M2U-bsp/blob/master/BPI-M2Ultra_Compiler_SOP.pdf
Please note, that PMPs are not reliable. More info here: https://linux-sunxi.org/SATA#Port_multipliers
I have tested it on BPI-M2U.