How are the device tree functions defined, modified and selected?

[JGWD]

I have a need to access both SPI A and SPI B buses at the same time. The below table shows the available “functions” that supposedly select between a set of available peripherals.

https://wiki.radxa.com/Zero/hardware/gpio

Where are these functions defined?
How are they selected?
How do I create my own (to use SPI A and B at the same time)?

Note that the Amlogic documentation also shows SPI A and B to be in different “functions”.

S905Y2%20Quick%20Reference%20Manual%20v0.7.pdf

Thanks,
/John.