uniphier: add a helper to get image_info

uniphier: add a helper to get image_info

In the next commit, I will have more usecases to get struct image_info
from image ID.  It is better to make a helper function at a different
layer.  I do not need the current uniphier_image_descs_fixup() since
the code is small enough to be squashed into the caller side.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>