ARM: tam3517-common: fix nand spl boot

ARM: tam3517-common: fix nand spl boot

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.

Cc: Pekon Gupta <pekon@ti.com>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Raphael Assenat <raph@8d.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tapani Utriainen <tapani@technexion.com>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>
Acked-by: Stefano Babic <sbabic@denx.de>