is your nand detected in bootlog? bl2/preloader can also be flashed in uboot directly to offset 0x0
you have to adapt to openwrt, but basicly it can be done like this:
https://www.fw-web.de/dokuwiki/doku.php?id=en:bpi-r3:uboot#nand_flash
example is for mmc, but it is easier to use usb (“usb start” before then use “usb 0:1” instead of “mmc 0:x”)
BPI-R3> mtd erase spi-nand0
Erasing 0x00000000 ... 0x07ffffff (1024 eraseblock(s))
BPI-R3> fatload mmc 0:5 $loadaddr bl2_nand.img #this should be the snand-preloader file
217712 bytes read in 24 ms (8.7 MiB/s)
BPI-R3> mtd write spi-nand0 $loadaddr 0x0 0x100000
you may also not erase your complete nand so add a range
mtd erase spi-nand0 0x0 0x100000
then do the other parts (ubi) from emmc-openwrt (or maybe before)
I need to install kmod-mtd-rw package. and I had to apply the last two P.S. in the tutorial.