The i2c port on the am335x does not natively support i2cslave mode by default…
There is a new kernel driver to support i2c slave features here:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/i2c/Kconfig#n113
it’s relatively new, so your kernel probally does not have it enabled. No one has previously documented it either on teh BBB.
Regards,