Convert these to rST format and add them to the index.

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

The DT spec demands a unit-address of a node name to match the "reg"
property in that node. Newer dtc versions will throw warnings if this is
not the case.
Fix all occurences in the FIT image example files where this was not
observed, to not give bad examples to the reader.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>

Added FIT_LOADABLE_PROP, so the user can identify an optional entry
named "loadables" in their .its configuration. "loadables" is a comma
separated list in the .its

Documentation can be found in doc/uImage.FIT/source_file_format.txt and
                              doc/uImage.Fit/multi-with-loadables.its

Signed-off-by: Karl Apsite <Karl.Apsite@dornerworks.com>
Reviewed-by: Simon Glass <sjg@chromium.org>