Before 9d0750064e (doc: Move external FIT docs into the main body), the
FIT property data-size was not a mandatory property and still it is not
expected to be set alongside the data property.
Move the data-size property to the "Conditionally mandatory property"
section, where it actually belongs.
Signed-off-by: Bastian Germann <bage@debian.org>
Reviewed-by: Simon Glass <sjg@chromium.org>