Since you mentioned that the A core started from the SD card(assuming using the BSP), then you could possibly try downloading the M7 application into SRAM by the “bootm7” command in u-boot，it is introduced in BSP user manual, in which you may reference it for the details.
I understand that you may want to directly download images to the sram to run the application on M7 cores, then from my opinion, there are at least the following ways:
Serial Boot mode enables application code to be downloaded to SRAM.
When boot from the A53 cores, use the command in uboot to download the binary to the SRAM to run it from M7 cores.
Use the debugger to directly download the image to the SRAM.
