mx6sxsabresd: Keep only one target

Currently there are two targets for the i.MX6SX SabreSD board:
mx6sxsabresd_defconfig and mx6sxsabresd_spl_defconfig.

This brings additional maintainance effort without a clear
advantage.

Keep only the mx6sxsabresd_defconfig one and remove
mx6sxsabresd_spl_defconfig to keep it simpler.

Also remove the SPL related code from the board file.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Peng Fan <peng.fan@nxp.com>