Kconfig: fix SPL_FIT dependency

SPL_FIT obviously requires libfdt in SPL, so let Kconfig express that by
selecting SPL_OF_LIBFDT.
Also make the actual options that users want (SPL signature and SPL FIT
loading) visible in the menu and let them select the SPL_FIT as a
requirement.
Also remove the now redundant SPL_OF_LIBFDT from those Kconfigs that had
it in for the SPL FIT loading feature.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
[Remove change from configs/evb-rk3399_defconfig]
Signed-off-by: Jagan Teki <jagan@openedev.com>