What processor/board/soc are you using?  The easiest path is to start

with a similar board and adapt the device tree source file (.dts in
arch/powerpc/boot/dts) to match your board with your devices.  You'll
need to add a node to the tree for each device you add.  If you post
your new .dts file to devicetree-discuss@lists.ozlabs.org for review,
then I and others will help you to craft it and get it into the format
that it needs to be in.