The spi on the low-speed expansion header is working fine.
In my case, we are connecting my board as a slave to the spi.
What I did was that just adding the node as a child to the spi node.
I didn't use the spidev in my case.
Even if you use it, whenever you do some transactions using the spi, you should get the signals on the low-speed expansion header.
For me without any debugging, it started working the basic stuff.
I used the spi max frequency to 4MHz.
Background: I am working on audio stuff and the codec should expose the registers in some manner (regmap i2c or spi).
Whenever I write something into the reigsters or read from them, I get the signals on the low-speed expansion header.