image: Rename fit_add_hashes() to fit_add_verification_data()

We intend to add signatures to FITs also, so rename this function so that
it is not specific to hashing. Also rename fit_image_set_hashes() and
make it static since it is not used outside this file.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>