spl: test: Add a test for spl_load_simple_fit()

As an example of an SPL test, add a new test for loading a FIT within
SPL. This runs on sandbox_spl. For this to work, the text base is adjusted
so that there is plenty of space available.

While we are here, document struct spl_load_info properly, since this is
currently ambiguous.

This test only verifies the logic path. It does not actually check that
the image is loaded correctly. It is not possible for sandbox's SPL to
actually run u-boot.img since it currently includes u-boot.bin rather than
u-boot. Further work could expand the test in that direction.

The need for this was noted at:

   http://patchwork.ozlabs.org/project/uboot/patch/20201216000944.2832585-3-mr.nuke.me@gmail.com/



Signed-off-by: Simon Glass <sjg@chromium.org>