“Where can I find the documentation for the device tree, which describe the modifications to enable all SPI Interfaces.”
There isn’t one, unfortunately :(
I’ve never had to enable all the SPI interfaces, it will just a matter of finding the numbers corresponding to the other SPI devices e.g. spi@3240000 and updating the device tree accordingly to reflect that. As I mentioned I’ve never used all 3 SPI (as it would require additional hardware to access them on the development board), but I don’t believe it would be that trivial. As ShaneCCC said, it will be a case of searching the kernel sources, in particular the hardware DTS for device tree information on the other SPI registers.