spl: fit: Only look up FIT configuration node once

The configuration node a sub node under "/configurations", which
describes the components to load from "/images". We only need to
locate this node once.

However, for each component, spl_fit_get_image_name() would parse the
FIT image, looking for the correct node. Such work duplication is not
necessary. Instead, once the node is found, cache it, and re-use it.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>