image: Display FIT timestamp when booting

The timestamp is shown in fit_print_contents() but for some reason not
in fit_image_print(). This seems to be an oversight, since it is the latter
which is used by bootm.

Add timestamp printing in this case.

(There is code duplication in these two function, for looking at in a future
patch).

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