Kconfig: Disable non-FIT SPL loading for TI secure devices

Non-FIT SPL image loading support should be disabled for TI secure
devices as the image handlers for those image types do not follow
our secure boot flow.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>