dts: mpc512x: prepare for preprocessor support

prepare C preprocessor support when processing MPC512x DTS files
- switch from DTS syntax to CPP syntax for include specs
- create a symlink such that DTS processing can reference includes

Signed-off-by: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>

arch/powerpc/boot/dts/pdm360ng.dts

@@ -13,7 +13,7 @@
  * option) any later version.
  */
 
-/include/ "mpc5121.dtsi"
+#include <mpc5121.dtsi>
 
 / {
 	model = "pdm360ng";