Update spi definition for Zephyr 3.2+ compatibility

[cairnm]

This branch updates the SPI definition to use &pinctrl which is a necessary change for ZMK using Zephyr 3.2 or greater. The update essentially replicates the example code in the ZMK docs.

I've tested this change and confirmed that it builds/runs when using the current ZMK main branch, and the underglow LEDs work as expected on my Hillside46.

[Matthew-PargaManasse]

I grabbed cairnm's changes and applied them to my build. I can confirm that it is functioning on my Hillside46.

[CaptainFlint42]

I also applied these changes and it worked for my Hillside 52.

[FilippeMafra]

I made the same changes and worked on my Hillside 48.

[LukeJohnson]

I have also confirmed that these changes resolve the build issue on my fork.

After merging the branch to my fork the build is successful.

Thanks for the fix @cairnm !