Thomas, has Arthur's reply answered your question? If yes, please click Correct Answer, otherwise please keep working with Arthur. Please do not leave the DI there un-attended, as we will close Di in 3 days, if nothing happens.
For SPI signals assignment, you can refer to Table 17-1 on the Reference Manual.
The hardware connections are basically the same for either master or slave mode. The difference is that i.MX28 SCK, MOSI and SSn0 are input pins in slave mode.