Please try following:
- set bootargs as
bootargs="ubi.mtd=rootfs root=/dev/ubiblock0_2"
Please also try following:
- rename mtd partition name "rootfs" to "ubi"
- drop bootargs line
I have tried adding support for the device of ipq806x, but I have no experience with the device of ipq40xx. So, I am not very confident about the second procedure...