Kconfig: Document the limitations of the simple SPL_LOAD_FIT path

The "simple" SPL_LOAD_FIT path is the most compliant with the format
documented in doc/uImage.FIT/source_file_format.txt. The other two
paths to load a FIT are SPL_LOAD_FIT_FULL and the "bootm" command.

Since the Kconfig menu is the most likely place for a new user to see
these options, it seems like the most logical candidate to document
the limitations. This documents the _known_ issues, and is not
intended to be a complete list of all follies.

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