Add a SPL default configuration file for mx6sxsabresd board.
Signed-off-by: Peng Fan
CONFIG_SPL=y CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/imx-common/spl_sd.cfg,MX6SX" +S:CONFIG_ARM=y +S:CONFIG_TARGET_MX6SXSABRESD=y