mx6: toradex: Remove custom CONFIG_SPL_PAD_TO definition

CONFIG_SPL_PAD_TO is already defined inside "imx6_spl.h", so there
is no need to redefine it in the board config files.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Stefan Agner <stefan.agner@toradex.com>