fit: Add empty fit_print_contents() and fit_image_print()

These functions may be needed in SPL, so add empty variants of them
if CONFIG_SPL_FIT_PRINT is disabled.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>