Initial work on keymap transforms for non-standard matrixes.

* Allows "gaps" for keyboards like Kyra.
* Allows keyboards with matrixes that don't map the logical key
  locations to have normal looking keymaps when defined.

app/dts/bindings/zmk,keymap.yaml

@@ -4,6 +4,10 @@ description: |
 compatible: "zmk,keymap"
 
 properties:
+  transform:
+    type: phandle
+    required: false
+
   label:
     type: string
     required: true