fdt: Show a message when the working FDT changes

The working FDT is the one which comes from the OS and is fixed up by
U-Boot. When the bootm command runs, it sets up the working FDT to be the
one it is about to pass to the OS, so that fixups can happen.

This seems like an important step, so add a message indicating that the
working FDT has changed. This is shown during the running of the bootm
command.

Signed-off-by: Simon Glass <sjg@chromium.org>