What device are you trying to control? Below is a project to control the ILI9341 displays from a BBB. I don’t have a pocketbeagle, you’d have to port the pin assignments. If you have a BBB, i suggest that use it to study my implementation before dealing with the port to the pocketbeagle.
this project does what you are looking for. it manipulates sclk, mosi (NOT MISO), DC and CS via symbolic substitution which makes the app level ASM readable. you can download the disk image and run from a uSD card … the project can be recompiled from source for easy modification.
would be interested in your progress, good luck
gomer