This is an exercise best done with a Lauterbach JTAG debugger. You don't have source, nor symbols, nor debug scripts, so you would need to be comfortable with assembler.
DDR setup/init/training happens early in boot, which executes from a region of static RAM.
In order to debug via JTAG you need to know what cores/types are involved in the boot chain, in order to attach and step through it all.
JTAG is exposed on the Dragonboard.