rockchip: rk3368: spl: move SPL_LDSCRIPT to Kconfig

With the new way of doing things (i.e. the hierarchical selection of
SPL_LDSCRIPT via Kconfig) in place, this moves the SPL_LDSCRIPT setting
for the RK3368 from defconfig back into Kconfig.

With this done, there should be no lingering cases of SPL_LDSCRIPT
outside of Kconfig files.

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