Add all the mtd partitions in the bootargs, I was getting the same error...
After adding mtd parition in bootargs I could get past it..
bootargs=console=ttyS0,115200 root=/dev/mmcblk0p2 rw rootwait ubi.mtd=5
Basically by doing so we tell the kernel to attach mtd5 to ubi.
If you are using mtd0 to mtd5 then add all of them in bootargs