Hello jupiter hce,
For i.MX platform, we provide documents for linux BSP, which includes u-boot and linux porting guide. you can download them here.
Embedded Linux for i.MX Applications Processors | NXP

Download one according to the BSP version you want to use.
Then decompress it, you can find i.MX_BSP_Porting_Guide.pdf, open it ,you can find how to port u-boot to customer's board.

Have a nice day!
BR,
Weidong