What I meant; what is the debian rootfs you installed to your mmcblk0p4 partition?
When your kernel boots does it find all it needs on the rootfs?

And you still did not answer; how far does the boot go, do you see anything at all on your device screen after ubiboot hands the execution to the debian kernel?

And another thing; have you checked that the debian kernel handles the CPU L2 cache initialization correctly? (See the section 4. of ubiboot README file...)