Hi,
There is no such document. You can download imx-mkimage
then you can find details that you need from /scripts/qspi_header and mkimage_imx8.c
make the target for, e.g., flash_evk_flexspi that you can find from soc.mak for reference.
and you can refer to RM, such as, Quad SPI Interface.
There is one Flexible Serial Peripheral Interface (FlexSPI) block that acts as an interface
to one or two external Quad SPI serial flash devices, each with up to four bidirectional
data lines.
Regards
Harvey