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>