lib: spl: differentiate between TPL and SPL for libfdt/of_control/of_platdata

This splits the compilation of code modules for TPL and SPL for
OF_CONTROL (and related) features between TPL and SPL.  The typical
use-case of this is a TPL stage that uses OF_PLATDATA at TPL and
provides full OF_CONTROL at SPL (e.g. on the RK3368).

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>