of: Documentation: change overlay example to use current syntax

The overlay implementation details in the compiled (DTB) file are
now properly implemented by the dtc compiler and should no longer
be hard coded in the source file.

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>