spl: spl_legacy: Add extra address checks

Check whether the loaded image or entry point does not overlap SPL.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Heiko Schocher <hs@denx.de>