commit f9095aac793aa8917ab9b915c5d449e6dc8d3d30, "mtd: nand:
omap: add CONFIG_NAND_OMAP_ECCSCHEME for selection of ecc-scheme"
removed CONFIG_SPL_NAND_SOFTECC from the tam3517 common config,
causing the spl nand boot to fail. Add it back, so derived
boards boot again.