spl: Allow SPL/TPL to use of-platdata without libfdt

At present libfdt is included in SPL/TPL if SPL/TPL_OF_CONTROL is enabled.
But if of-platdata is in use this is not required. Update the condition to
avoid building this extra code. This ensures that if a libfdt function is
used it will produce a link error rather than silently increasing the
build size.

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