spl: Kconfig: migrate $(SPL_TPL_)LDSCRIPT to Kconfig

Now that we have split up SPL_LDSCRIPT into a SPL and TPL variant and
have started to use the TPL-variant for the RK3368, it's time to clean
up behind ourselves: move both variants into Kconfig and remove them
from the whitelist.

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